Personally, I don't use a split shot on my tippet or leader and instead, when I tie my nymphs, I weight them, instead. (not liking the idea of "polluting w/lead", I use a lead free weighting material to do this step).
I don't like "bobbers", so neither do I use "strike indicators". I was taught to use 'feel" in my fly line and "watching tip action".
I don't advocate, or, suggest this method for anyone it's just the way "I was taught' and it's always been very productive for me.
In answer to; "two nymphs being more productive than one"?
Yes, possibly so. But, then, 3 would be more productive than 2 and 4, would be...................and so on.
If you want to use the indicator style, of nymph fishing, tie on a bushy hackled dry as your "indicator" and who knows............ you may get a very surprising, "double hookup"!? I've seen other fly fishers, many times, fighting two fish at once this way,(which almost always, ruined their leader/tippet set up), but 99% of the time......if the fish are there, they'll hit one fly or the other!