To have full redundancy in both buildings, the ideal is for you to have an MX in each building configured in Warm Spare.
It is advisable to have at least two Links from different ISPs and preferably the router of one ISP is in one building and the other in the other building, this way you can guarantee redundancy in the event of failure of one of the ISPs or one of the MXes.
Remembering that there must also be redundant fibers to interconnect each building.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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