manual transfer switch - 2 main panels
Same meter, then yes, you can, But --

A wise man will not use all the circuits in a new breaker box.

The day you find the one you forgot about, there is no place for it to go.

The prewired tail on the reliance manual boxes are just not designed to work the way you want.

Each circuit has to run from the wire that is under the breaker, to the reliance box and back to the breaker. So you will have to run more than a few wires between the panels.

It is easier to move 3 breakers from one box to the other than to try and wire 3 transfer circuits into each box.

It is wiser to spend the $250 - $300 and get another 6 circuit box. Then put one on each panel, and you will have spare circuits in each panel. You will find a use for some of them in the future.

It will also make it much less difficult to change to 50 amps of back-up, if/when you outgrow the 30 amps that only the one box will give you.