This seems to be reported by NASA - NTRS in a document on Orbiter-Orbiter And Orbiter-Lander Tracking Using Same-Beam Interferometry [Published 1992 Acquired 2013]:
Two spacecraft orbiting Mars will subtend a small angle as viewed from Earth. This angle will usually be smaller than the beam width of a single radio antenna. Thus the two spacecraft may be tracked simultaneously by a single Earth-based antenna. The same-beam interferometry (SBI) technique involves using two widely separated antennas, each observing the two spacecraft, to produce a measurement of the angular separation of the two spacecraft in the plane of the sky. The information content of SBI data is thus complementary to the line-of-sight information provided by conventional Doppler data. The inclusion of SBI data with the Doppler data in a joint orbit estimation procedure can desensitize the solution to gravity mismodeling and result in improved orbit determination accuracy. This article presents an overview of the SBI technique, a measurement error analysis, and an error covariance analysis of some examples of the application of SBI to orbit determination. For hypothetical scenarios involving the Mars Observer and the Russian Mars '94 spacecraft, orbit determination accuracy improvements of up to an order of magnitude are predicted, relative to the accuracy that can be obtained by using only Doppler data acquired separately from each spacecraft. Relative tracking between a Mars orbiter and a lander fixed on the surface of Mars is also studied. Results indicate that the lander location may be determined to a few meters, while the orbiter ephemeris may be determined with accuracy similar to the orbiter-orbiter case.
Introduction:
The planet Mars will undergo an intensive program of unmanned exploration in coming years. In 1993, the Mars Observer spacecraft will begin to map the Martian surface. The Commonwealth of Independent States (former Soviet Union) is scheduled to launch missions to Mars in 1994 and 1996, in which a number of landers and balloons will be deployed. The U.S. Space Exploration Initiative encompasses a broad range of unmanned and manned trips to the Moon and Mars, which will involve a variety of orbiters, landers, and rovers. The presence of multiple spacecraft at Mars could provide the opportunity to perform extremely accurate Earth-based navigation by using a radio metric technique called Same-Beam Interferometry (SBI) [1,2,3,4]. SBI measurements provide information about spacecraft-spacecraft separation in the plane perpendicular to the Earth-spacecraft line of sight, naturally complementing the position and velocity information obtained from Doppler and ranging measurements. SBI could prove to be a valuable navigational tool supporting future Mars missions, potentially aiding in the final hours of Mars approach navigation, enabling improvement of Mars orbiter ephemerides and providing measurements of the relative positions of landers and rovers with an accuracy of several meters.
Short example explanation of double-differenced delay:
SBI observables involving two spacecraft and two Earth antennas are obtained by accumulating spacecraft signal phase at each of the two Earth antennas. If the signal transmitted by each spacecraft has frequency $f$, then the double-differenced delay $r$ may be expressed as
$$r(t)=\frac{(\phi_{12}(t)-\phi_{11}(t))-(\phi_{22}(t)-\phi_{21}(t))+b}{f}$$
where $\phi_{ij}(t)$ represents the phase of the signal transmitted from spacecraft $i$ and received at station $j$, and $b$ is an unknown integer. Because of the unknown integer bias, an SBI measurement does not directly provide the doubledifferenced delay. However, SBI measurements obtained continuously over a time interval share the same bias and thus provide a precise measure of temporal changes in the double-differenced delay. Given sufficient a priori information about the spacecraft states, the integer cycle ambiguity can be resolved and the absolute double-differenced delay determined. Typically, a priori information about the spacecraft states is not sufficient to determine the integercycle ambiguity, and the SBI phase bias must therefore be estimated. If the sigma on the SBI phase-bias estimate is smaller than 1/6 of a cycle of carrier phase, the integer-cycle ambiguity can be resolved with 99-percent confidence.
Full document pdf can be found here
Yet another document on In-Situ Navigation and Timing Services for a Human Mars Landing Site Part 2: System Design and Simulations [Published 2018]:
Improved OD using SBI was demonstrated with the Pioneer Venus and Magellan orbiters at Venus [13][14]. SBI was also used or proposed for use in various deep space missions including lunar missions [15 – 18]. For multiple orbiters at Mars, SBI could be used to improve OD while requiring fewer Earth-based tracking resources. All spacecraft within Mars areostationary orbit would be visible within the 1-mrad beamwidth of a 34-m BWG antenna
at X-band. All signals would be acquired simultaneously.
Short evidence on proposal with no follow-up found yet on Same Beam Interferometry on Mars for obtaining information on the interior:
A mission involving Mars’ landers will almost certainly envisage a direct-to-Earth radio system for tracking telemetry and command. If the mission entails a network of landers, accurate geodesy and geophysics experiments can be enabled by a tracking
configuration known as Same Beam Interferometry
(SBI). SBI was proposed in connection to network of landers for the moon and recently for Mars.
Additionally, there seems to be a book search here on Radiometric Tracking Techniques for Deep-Space Navigation that supports the future use of this technique [Published 2003]:
The concept of differential tracking for angularly close sources is well established and has been applied to numerous astronomical problems. Furthermore, improved orbit determination using using this technique was demonstrated with the Pioneer Venus and Magellan orbiters at Venus. The next-generation VLBI system implementation, described in Section 5.2, could provide the means for operation use of this technique. As more spacecraft begin operating at Mars, SBI could be used to improve orbit determination while requiring fewer Earth-based tracking resources.
To conclude, all sources seem to support the future use of SBI technique. However most of this articles/papers are not new and recent, so in the mean time I'll keep looking :) Maybe they are already putting SBI into operational use!
The NASA sources seems to state that SBI has been used and tested for hypothetical scenarios, but I'm not sure about full operational use.
