We have a similar setup, two pairs of MX450s in different DCs. It is possible to setup 'secondary concentrators' for tunnelling SSIDs, screenshot below: We split the connections to different HA pairs of MX450s to spread the load, but have secondary concentrators setup in case of DC failure. ... View more

Focus on the bit that @Ryan_Miles and @ww mention first.   Each MX needs its own IP address.  You need at least a /29 from your ISP to make this work using a single ISP connection.  You could also consider getting a second /30 connection from the ISP, one for each MX.   You could also plug the /30 into only the primary MX, and then get something like an MG21 cellular gateway, and plug both MX into that. https://meraki.cisco.com/product/cellular/integrated-antenna/mg21/  ... View more