If you use Routed IPsec (VTI) then you would have an IPsec gateway for each tunnel that you could use with a gateway group. Though because VTI doesn't support reply-to it may not be as ideal as it could be, the return traffic would only take one of the two WAN tunnels.
You can pull it off easier with OpenVPN (tunnel up on each WAN, assign the interfaces, use gateway groups + firewall rules only on the assigned interface tabs)