2 BR1 Mini devices 4 Sims 1 connection

So I am looking to get 2 of these devices working together on failback when data caps are met on 1 device between both sims, to go to the 2nd device, and vise versa once the data reset happens. Basically I was asked to do the following config before sending off the devices to a customer. I am confused and still trying to learn.

Router 1: New device to configure to add into the 2nd router

  1. Configure Failover, such that SIM1 cuts off at 295GB and switches to SIM2.
  2. SIM2 will cut off at 295GB.
  3. Configure the billing cycle for each
  4. The connection will leave through LAN 1 of Router #1, and enter Router #2 in the WAN1 Port

Router 2: This is the router the customer has in place already, It will need a new update to start with WAN 1 Port. but primary wifi connection is this router.

  1. Configure Failover priority as
    A. WAN1 Port
    B. SIM1
    C. SIM2
  2. Set the router to failover from WAN1 to SIM1 when the connection is lost.
  3. Set Router to failover from SIM1 to SIM2 when SIM one reaches 295GB.
  4. Set SIM2 to cut off when it reaches 295 GB
  5. Configure the billing cycle for each
  6. IMPORTANT Set Failback criteria to be when WAN1 restores connection.

I need some clarification if anyone has done this before, connecting 2 BR1 Mini together using 1 sim at a time until data is capped and the 2nd router being the router to provide all the data.