2 USB dongles, 2 Asus Routers, Dual Wan Setup help

Hello, I am hoping I can get some help with the setup I would like to have:

1 87R running latest Merlin firmware
TM AC 1900 with stock firmware

Due to living in an extremely rural area, I have LTE as my primary Internet provider.

I have 2 USB dongles, both of which work fine on each of the routers.

Since both routers only allow 1 USB WAN connection, I currently have to use them separately, with clients connecting on separate networks.

I would like to setup the 87 in a dual WAN in...

2 USB dongles, 2 Asus Routers, Dual Wan Setup help