I encountered a similar situation recently. Given there was a LAN connection between the 2 buildings and there was no technical reason for them to be separate, I merged into a single LAN (with VLANs etc where necessary), which allowed me to put an MX95 in each building and configure HA. Don't get me wrong, it wasn't a simple task and required some detailed planning, but in essence that's what I did. Is this an approach you could take and get 2 MX's of the same model to give you the redundancy you need? If you kept existing subnets then you could even route each building over its existing ISP but have the backup of the ISP connection in the other building.
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