My standard ducking is a comp on the ducked track and a send from the control track to the sidechain comp.
How does it work with several trax?
Surely we can use a buss for this?
What would be the settings?

If you want to use several tracks as the keying tracks to duck one, then you can just insert sends on those tracks to the sidechain input. You could send them all to a buss and then key from the buss but it wouldn't save any work, in fact it would create more because you'd still have to put the sends onto the tracks (to the buss) and in addition create a send from the buss to the sidechain input.
 
If you meant to duck several tracks at once, you could output them all to a buss and then duck the buss but I can't envisage that's something I'd want to do. Ducking several tracks at once is probably going to create an audible "pumping", unless that's the intended effect of course.

"If you meant to duck several tracks at once, you could output them all to a buss and then duck the buss but I can't envisage that's something I'd want to do. Ducking several tracks at once is probably going to create an audible "pumping", unless that's the intended effect of course."
That was it, thanks.
The tracks to duck were left and right rhythm tracks in this case. Ducking one would throw off the balance. I am using a soft knee to minimise the pumping.