The TCAS warning says: " DESCEND, DESCEND" and immediately after the crew receives: "TERRAIN, TERRAIN", what would they do?
TCAS (II) does not give the descend RA below 1200 ft AGL while climbing or below 1000 ft AGL while descending.
Image from FAA Advisory Circular 20-151A Table 2
In case such an unlikely event happens (and is not an equipment malfunction), the best possible maneuver is to climb away from the incoming aircraft i.e. turn and climb to the side of the previous route.
I would pull up and once in a stable climbs I would turn in either direction in this situation. If I get a warning to descend it is to prevent a conflict with another aircraft, whereas a terrain warning is to prevent impact with the ground. The ground is much bigger than another airplane, so the lower risk is collision with an aircraft. In other words, if I descend I will certainly hit the ground, if I climb I may hit another aircraft, so climbing wins. Turning in either direction will hopefully decrease the collision risk.
It would suspect this is an unlikely situation, TCAS should not give an instruction which would put you into another conflict.