Since it appears you are connecting PC-PC and not PC-vpn appliance your issue is likely NAT. You are somewhat lucky it even works with a single VPN, the router needs a special feature to support this.
Without getting very detailed into why it does not work it is limitation of IPSEC and PPTP. This is very similar to why you cannot have 2 servers both using port 80 behind your router. You can only port map the outside port address to single machine. It much more complex for VPN, it is partially a mapping type issues but it is also in the security protocols themselves that detect sessions from duplicate ip.
Pretty much the only way to fix this is to put the VPN on your router itself. Since this has direct control of the real IP address there is not a NAT problem. The other way to fix this is to run something like OPENVPN that supports running VPN over TCP. Generally they run it over SSL/TLS port 443 so it runs like HTTPS. This is much more tolerant but if you needed multiple machine in your office running VPN to multiple machines on the internet you would have to use different TCP ports for each.