You have done some research and are doing very well for being so new!
I get what your saying and really that is something we all wonder about when there is more than one glider in a cage. It's not something we can really control.
One thing you can try is offering the liquid part as a treat during the day to the one that you notice eating more of the salad, just put like a half a tablespoon in a cap or something shallow and offer it to him.
And vice versa offer salad as the treat to the one eating the staple, offer a few pieces of fruit or veg from the salad it should thaw pretty quickly if your just taking a few pieces out.
IF you use the diet as treats during the day, reduce the portions by the same amount you used for the treat at night for normal feeding time.
It doesn't always work they may not be hungry enough to eat much of it during the day, but if it does you will at least know they are getting some of both parts of the diet. It just happens to be at a different time of day.