This is *probably* way over-simplifying things, but we ended up with a similar situation at a couple sites. The way we handled it was using two unused VLANs to stretch the ISP connection from building A to building B, and vice-versa. Talked to the provider(s) to have them widen the subnet a smidge to handle 2 active devices, set IP's on WAN2 on both devices. So, we have ISP auto-failover for both buildings, but not warm/spare in the event we lose one of the MX's; The idea being that one of our regional spares could be easily swapped with the 'dead' MX if/when needed.
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