You can already do that - but better. Connect two different upstream providers to the two WAN ports.
BGP advertising a default route only verifies connectivity between you and the ISP is working.
The MX verifies connectivity all the way back to the Meraki cloud - so an issue within an ISP or their upstreams will now be detected.
Using email as an example, you can point the DNS at the pair of IP address (one from each provider).