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That sounds right. The primary and secondary WAN interfaces should end up in separate VLANs on the upstream or WAN switch. Both device's X1 interfaces and the X1 gateway should be in one group and both device's X2 interfaces and the X2 gateway should be in another group.
Basically it should be logically divided such that both of the WANs do not see each other on that switch. That is a normal topology that I have seen.
Technical Support Advisor, Premier Services