I'm making this new design and I just need to know if I can install NTC in parallel at the moment I only add one as optional
Capt is correct. Parallel problem is one will start to get hotter than the other steering more current to the hotter one which will make it even hotter steering more current to it... effectively getting you one NTC instead of two. Not sure of the actual requirement, but I put more space around an NTC. They do get hot. Burn your finger hot.