Can you wireless link trunk VLANs?
If so, you could create a VLAN and trunk WAN2 of each MX into the back of the MX at the other site.
If your ISPs have given you a block of routable addresses (like a /29) you could run the ISP circuits into a VLAN at each site, and trunk it across both sites, and then both MX could plug into both ISP circuits.