I, like everyone else, am looking for that perfect hunting load for pheasants. I would like to reload some 12 gauge shells with a mixture of #4 shot and #7 1/2 shot.
Sure it can be done, has been done, but there is no advantage to playing with duplex loads that has ever been shown. I've averaged over 35 cock pheasants a year from Illinois ditches, waterways, and hedgerows for over thirty years. Naturally, I have some opinions.
Neither #7-1/2 or #6 shot has any place in an effective pheasant load-- too many runners. If you want to be certain of picking up your bird, you need to break a wing and a leg. #4 or #5 shot does that more reliably than #6 or smaller. Nickel plated, #5 shot is the best compromise I've found-- using #4 late season, starting right about now.
This should come as little surprise to waterfowlers who used #4 or 5 shot on decoying ducks when lead was still allowed. Far more exposed bodies than on a pheasant that hops up, head down, wings pumping as hard as he can to get away in a hurry.
The next bit of advice (correctly priced at free) is to pattern your hunting loads. That at least gives you an idea of the pattern percentages you might expect out in the field-- in your gun.