Two separate and distinct IAP clusters (that is two IAP clusters that were setup separately, on different L2 networks, etc) will not join each other's clusters via mesh. There is a hashed Mesh Cluster ID that is established upon the setup of any new VC/IAP Cluster that is then pushed to any IAPs that later join that cluster. If Business 1 sets up IAP cluster 1 and it gets mesh cluster ID 12345, then if business 1 then adds another IAP to that cluster via L2 and it shows up in the IAP VC, then it will also get that same mesh cluster ID. Then if Business 2 sets up their own IAP cluster on their own L2 network, their cluster may get ABCDE as their mesh cluster ID.
Both businesses would need to first disable the extended SSID to enable Mesh.
If Business 1 IAP loses the wire, it will search for another AP broadcasting the mesh cluster 12345. Business 2, if an IAP loses the wire, will look for ABCDE. Business 2 IAP won't join Business 1 IAP (unless they were all originally provisioned as the same VC under the same cluster, which would be rare I hope).