A single basic radar antenna can't tell you much. The most basic radar setup involves a fixed sending and receiving antenna and some software and drivers. Modern radar then uses a frequency ramp up or "triangular" pulse which it sends from the tx antenna (for example you sweep from 24 to 25 GHz over a short time). Then, you analyse the signal that returns. The time of flight can give you information on the objects range and the amplitude or signal strength gives you how radar reflective the object is. Then, you can analyse doppler shift and tell if the object is moving towards or away from you. The problem is, if you are only using one radar device (tx/rx pair) you only get two dimensional data. The rover can tell that it's above an ice pocket but it can't tell you if the ice pocket gets thicker to the left or right. This is why radar systems often have multiple antennas pointing in different directions or a spinning element.
Essentially, with one radar, the rover would generate an axis of data extending from the radar downwards. As the rover then drives, it drags this axis through Mars's surface and generates a 2d plane of surface data perpendicular to the ground. By having two radars perpendicular to the driving direction, the static rover creates a "plane of data" (which is coincidental with both of the single radar's axis). When the rover drives now, it "drags" the perpendicular plane through the ground and thus generates a 3d map of the ground. See arrangement of radar units here:
I've made some animations to explain what I'm talking about:
In this animation, a narrow beam radar is aimed downwards and detects an object. The rover does not know where in the "possible object location" area the object is but since the angle of the radar is so small, it can assume that it's within a small area.
Here the rover is using a wide beam to cover a lot of underground area. It can detect that there is an object and it can tell how far the object is away from the radar but it can't tell the angle of the object in relation to the radar.
Here the rover uses two wide beams and because it knows the distance that the object is from each radar and the distance between the radar units, a little trigonometry can find exactly where the object is underground. By driving forward while preforming these scans, a 3d image of the ground can be generated.
Note: This is a big simplification.