You have two VPN servers. One is located in the
main corporate office and the second is located at the backup site. You want to
provide centralized authentication and logging. What will you do and why?
If you have two VPN clients and their "advertised sets of addresses" do not overlap, then chances are that they will live together nicely at the IP level: each will grab the packets for its own virtual network, leaving the other packets untouched. However, they might also fight for the "interception resources" which may result in the first VPN client to be wholly deactivated.
On the other hand, if both VPN advertise overlapping sets of addresses, then trouble is pretty much guaranteed. If you are lucky, the second VPN will refuse to run with an explicit message. Otherwise, one of the clients may take precedence, possibly intermittently, and things will be weird and confusing. Possibly, one VPN server will receive the packets which were due for the other VPN, thus incurring a severe data leak.