Try putting the words AND or OR in searches with two terms in it, like 2% OR Milk or 2% AND Milk. That used to work in versions of PHPBB 2 and should still do in 3.
The thing with combo exercises, sounds fiddly, as some may do custom combos, that are not generally the "norm", but possibly there could be a way to tag a second selection into the space provided somehow.
I've not used those food and training logs, so I'm not fully au fai with the way they work.
As ever though, feedback like yours is appreciated and if yyou need any other assistance let us know. We're here to help
Take care and best wishes with your goal(s).