Multi sowing a module

I think it could work if you used something like a mesclun mix, then it'd be sort of like those living salad mix trays. For bigger plants which you don't harvest regularly to keep small, I think the faster growing plant will swamp the slower growing ones. Onions are very slow to get going and would disappear next to a lettuce.

I do interplant spring onions between lettuce and other salad leaves, but only in the middle point of a square foot and when the onions are tall enough to compete for light.