Hi there,
Great You are well on your way 
In that scenario you can have one Xiao as a BLE Central(server)Knobs and buttons 
the other Xiao as a BLE Peripheral (slave) with the connections to Models LED’s & Motor shield/hat.
pressing buttons to connect and disconnect from any number of peripherals and there is a mode for more than one at the same time. LQQk at Msfujino’s (i call) BLE Nija is a member here has posts to that configuration definitely next level read.
You can use the nRF_Connect for desktop and the Nordic $9 dongle connected to the computer as I do instead of phone. (until my app is ready for it…) absolutely makes BLE stuff idiot proof for testing.
looks like this;
comes like this;
No brainer
HTH
GL
PJ