I've been grinding RTTS with my two-way player, but I'm a little confused and want to make sure I'm doing this right.
I've split my progression between two archetypes: spark plug and painter. Should I be doing this? Or should I only be focusing on the painter? I want my player to be able to hit AND pitch once he's ready.
It sounds like you're doing it correctly. As your player is not going to need the benefit of the Painter archetype during days you're not pitching, nor will you be able to level Painter when not pitching, having a second loadout with Sparkplug appears to be the way to go.
I currently have three loadouts, though I only use two now: Pitcher (Filthy), Fielder (Sparkplug), and DH (Slugger). I now use the Fielder loadout even when I DH because I didn't want to split hitting archetype progression across Sparkplug and Slugger. Fielder also has equipment that boosts stats relevant to hitting and arm strength that Pitcher wouldn't benefit from, but equipment stats don't seem to count in RTTS right now anyway, though that's a discussion for a separate post.