Hi Fernando,
you can just configure two vpn's each with it's own ST0.x interface, so routebased vpn's. Of course you speccify different outgoing interface on the dual isp side en different destinations on the otherside. Dont forget to add st0.x to a zone and allow trafiic in and out with a policy.
In the routing table you can decide what to do two routes with different prefenece: (one with qualified next-hop): active passive. With a stateless filter and load-balance-per-packet action (which in pact is per flow, it's just called per packet) active active.