Speaking from gleaned knowledge rather than experience.
You can't have two tunnels to the same destination on the Meraki side.
The way to implement redundancy for a non Meraki S2S connection is to have a second WAN link on the MX with a separate public IP.
On the ASA, you then configure the backup IP for the tunnel to point to the MX's second WAN link.
In this way, is there is an issue with the primary link, the tunnel will be re-established on the backup link with a very minor impact to network traffic.