I make a lot of mixed fruit jams. This year I made banana-rhubarb and black raspberry-nectarine. I do it when I don't have enough of the main fruit. I look at the recipes on the Certo directions, find the appropriate recipes for each fruit, and adjust the sugar accordingly. For example, if raspberries require less sugar than peaches/nectarines, because I am adding nectarines to the mix, I add a little more sugar. It's never a lot, and I've never had a failure this way. I assume you could do the same with jelly.