There are parts of your question that I do not understand, like where you say that you have vpn connection between ISP1 and another site. Do you mean that the VPN is between the ISP and the other site (and just passes through your router) or do you mean that the VPN is between ISP1 and your router and processes traffic for the other site.
In any case the fundamental question should be easy to answer. It is very possible to have two VPN sessions on the router as long as the destinations are different.
HTH
Rick
HTH
Rick