That would be quite the complex setup. It would seemingly be far easier to place another mx at site 2 so both have primary wan uplinks. You can then set the point to point link as a secondary WAN uplink. Assuming you have the appropriate routing that would achieve the uplink redundancy you're looking for. But going strictly off what you're asking, unless your point-to-point link is providing some sort of stretched layer 2 overlay, I don't see how you can make it work.
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