Placing "overlapping" PCB antennas
I'm not intending to use them at the same time, I'm going to populate one of the SOCs, not both. I want to do this because I'm trying to migrate to the second SOC, but do not want to do it in one PCB spin.


This is the key part to this question.

If this is the case, it is likely far better an idea to use one antenna and use something like a jumper resistor to switch the right SOC to the antenna. 
Alternatively, if this is a prototype board, you can connect both to the antenna, and expose the copper near the junction between the two (so no silkscreen/soldermask) and just use a knife or such to cut the trace to the one you don't want to connect.