Your combined yield would be the finished weight divided by the starting weight of EVERYTHING. But you're trying to estimate, so, take the estimated yield of each individual like 80 percent of eggplant plus 60 percent of pork and then divide it by the initial weight of both ingredients. Say you're making pork and eggplant goulash and you have a pound of pork and a pound of eggplant, 60% out of a pound of pork is

0.6 x 16oz = 9.6oz

and 80% yield from your peeled eggplant is

0.8 x 16oz = 12.8oz

12.8oz + 9.6oz = 22.4oz

22.4 / 32 = 0.7 or 70%.

If your portion is 8oz per person then this will feed about 2.5 people. Since you know now that a pound of pork and a pound of eggplant will feed 2.5 people and you want to make it feed 4, you divide the original amounts by 2.5 and then multiply by 4. To convert the recipe to feed 4...

16oz pork divide by 2.5 = 6.4oz

multiply by 4 = 25.6oz or about 1lb 10oz

Same thing with the eggplant, you will need 1lb 10oz.

if you're making a batch with 50lb pork and 50lb eggplant (you'll get 70lb yield.

If you're given a bulk recipe the yields should be given, although this can vary (a lot) if you have different people making the recipe. But you're trying to estimate, and this makes things more difficult. The ONLY way to get a reasonably true yield figure is to average the yields over a period of time, like have someone make the recipe once a week for six months.

Kuan