Two targets is a great way to go.
Place them at whatever distance you want to shoot, right-left as quick as you'd need on game and that can be as quickly as you can regain sight picture.

No good reason to shoot an accurate double rifle at *only* very close range, and it's good to shoot (for record) both barrels on paper at all ranges you'd shoot.

200 yard shots around here are not uncommon at all, more so they're the typical minimum on most game-animal size targets of opportunity.

For the 200yd targets I'd just run one pair R/L and call it done.
A super quick second shot from a light recoiling and accurate ~30cal double can be very useful.

On the issue of clean VS fouled barrels, that will depend on how the rifle shoots - does it make a difference with that rifle? If so, then I'll test with fouled barrels - as I'd then also hunt with fouled barrels.

Cheers
Tinker