Why are you talking about PTCs, on BOTH the positive and negative sides ?
(Assuming the cell remains insulated, inaccessible, to the rest of the circuit, except its terminals, obviously).
This assumption can't be assumed. The battery will be on the underside of something and some parts of the battery terminals will be accessible.
It makes more sense now.
In general, it is usually best that the battery(s), its holder and any wires connecting it to the main PCB(s). Should be insulated, physically protected, encased etc etc. Enough so that the batteries, are safely away from any kind of accidental short circuits or something.
I'm not exactly sure what your project would look like, or do. But it does seem to be beginning to sound like you want to fix one or two PTCs, to make up for the fact that the battery installation is rather/potentially weak and sustainable to shorting out.
The thing is, occasionally/rarely (in theory), the safety circuit can fail. So if that happened in your setup, not much else would need to go wrong, for there to potentially be a fire hazard, created.
Really everything should be sorted out, the final step being the last compile and download.