VPN clients by their design try to take over the networking on a machine. Lets say you managed to do what you propose and have 2 vpn clients up to 2 different customers. You have now in effect connected the 2 networks together with no security other than what your machine provides. The only reason it has not caused a issue is the VPN are stupid and you would have to do more to route the traffic together but it is very possible if you do it intentionally.
For this reason most people set the options to prevent you from running more than one....then again IPSEC is pretty particular about things and will detect other vpn as a attack and will disable it.
The best way would be to install very minimal virtual machines on your machine and run a vpn client in each.