On an ASA, you can only have one default route (0.0.0.0/32) going to a single interface, but you should be able to use the other interfaces for VPN connections.
For example:
Interface outside1 IP Address: 1.2.3.4/24 with default gateway of 1.2.3.1/24
Interface outside2 IP Address: 2.3.4.5/24 with default gateway of 2.3.4.1/24
You would have your normal default route of route outside1 0.0.0.0 0.0.0.0 1.2.3.1 and you would point your corporate VPN tunnels to the endpoint peer address of 1.2.3.4.
You would then have routes for your partner sites going out interface outside2. For example, a partner with IP of 4.5.6.7 you would: route outside2 4.5.6.7 255.255.255.255 2.3.4.1.
Point the partner VPN endpoints to build a tunnel to your external address of 2.3.4.5.
Make sense?