Might be able to use Gaia Policy-Based routing...(just a thought)
You can define x2 default routes for ISPA and ISPB, However you still need to define the cluster topology information correctly.
The Policy-Based routing table will also have to be manually defined for each connected network. (bit of a pain if you have a few).
As an example...(I have done this for a customer that wanted a DMZ routed out a specific ISP, so can't see why this would not work in the same manner)
Client A - 10.100.0.0/24 would always route out of ISPA
Client B - 10.200.0.0/24 would always route out of ISPB