You'd need what with Lancom is called N:N NAT: translate both subnets to each other, so that one thinks it's talking to 192.168.11.0/24, the other to 192.168.12.0/24 (or any other unique range).
It's not easy to set up and can be a major pain to debug, so if there's any chance I'd rather change the subnet address on one side and route them without NAT.
If there's no easy way to change a subnet, maybe you can get away with adding another, unambiguous subnet on top on each side, so that each PC has two IP addresses, one of which can be routed from the other.
Changed or at least additional unique addresses would also solved the problem with the single NAT router: you assign the default gateway only to the unique address binding, then you can NAT to WAN without problem. Alternatively, you'd need at least one additional NAT router before the central one.