As was mentioned, 1v1 FW trains IS. 1v1 Guards does not train IS, but does train JS a bit faster. I prefer 1v1 FW, especially on guards as training IS is harder than training JS.
Also, the difference between single and multiple position training is pretty big. 2-position training is somewhere between 60%-75% of one position training. 3 position is around 50% of single position.
Doing the math though, 2 position training will improve your team faster (because you are training double the amount of players). This is something I am experimenting with in Utopia, but that's mainly because my trainees are mostly starter and star potential. If I had an MVP or better, 1 position training would be preferred.