warm spare is not active active.
you could run both providers to both mx's if you have enough IP addresses between the provider and your MX's .
https://community.meraki.com/t5/Security-SD-WAN/Does-MX-device-can-support-dual-active/m-p/64242#M16...
so the WAN ports can be both active, but not Both MX's (in warm spare) at the same time