Two satellites is never enough. In the picture below two satellites can cover the surfaces of Mars between the pairs of tangent lines enveloping each satellite, but a belt (shown in blue cutting through Mars) is left uncovered because it is outside the region between the pairs of tangents. On Mars or any other round body, you always need at least three satellites to have at least one of them communicating at every longitude around the Equator.
Moreover, the method of multiple areostationary orbits cannot work at all latitudes. This is basically because the satellites in this method must be centered on the Equator and the tangents I have drawn can never reach the poles from there. In practical Martian terms this means we would not be able to rove the polar ice caps freely with an areostationary configuration. There are configurations that cover the entire planet such as this approach with four satellites, but they do not use the areostationary orbit.