As many skills are afkable you can train 2 side by side, they do make training slightly more challenging and slightly more fun, it's by far easier training one account but if you can afk 2 accounts at the same time why not? It does however, make you limited to the skills you can train at the time of training both though, as it is impossible to effectively train agility on 2 accounts symaltaniously but training fishing on 2 is perfectly possible. In my case I do have another account for when I get the urge to train combat (I regard myself as a non combat skiller) I can do it without gaining a combat level on my main, or I go to sc and get some tools along the way.