Do you have static routes set up between the subnets? I think it will be very difficult for your subnets to communicate especially if they share IP addresses.

For example, if VM1 IP was 192.168.12.50 in it's subnet, and VM2 was the same however in it's own subnet, how would the router ever know which one you are trying to route to?

So first off, I'd change one of the subnets to a slightly different value, 192.168.13.0/24 if it's available.

Through the openstack network interface, share the 192.168.12.0/24 network and create a port on the 192.168.13.0/24 subnet. After the port has been created, find the IP of the port and add a static route to the 192.168.12.0/24 that signals for the traffic intended for the .12. network goes through the port created and found above.

This is how we allow inter-project access through openstack. You will also probably have to set up security groups to allow access unless you have your instances wide open.