Spark at least has business plans that have data pools which you can add data-only SIMs too.

Is 2degrees of any interest? We have a Private APN so we can do data pools easily there too, with Public IPs or all on private IPs so they can communicate (plus NAT to the internet)

OK, thank you for the positive reply. I'm not too precious in terms of provider, with the only preference is to couple two (maybe three) providers to get the best of both sets of coverage; where one might be weak or non-existent signal the secondary provider makes up for.

Shindig:

 

OK, thank you for the positive reply. I'm not too precious in terms of provider, with the only preference is to couple two (maybe three) providers to get the best of both sets of coverage; where one might be weak or non-existent signal the secondary provider makes up for.

 

You're going to have to handle that as 2-3 separate SIMs with failover. You won't get a single SIM and router and single bill via two different networks AFAIK.

The idea is two pools of data, with two different providers with multiple devices taking from each pool. The devices would auto failover between the two, with the connection being seamless to the end user. 

I'd like to offer unlimited data to the end user as well, so doing calculations around a 30 day theoretical max usage versus real world based upon reports from early 2020s that suggest average usage.