Here are the codes:
FOLLETT:
2P: Hold A, B +C
1P: ↑, ↑, ↑, ↑, B,B,A,A, ←, ←, →, →, C, C, C, ↓, C
ROSA:
2P: Hold A, B, +C
1P: C,B,A,C,B,↑,A,→,C,←,B,↓,C,B,A,A,A
UNLOCK CANYON STAGE:
2P: Hold A, B, +C
1P: C,C,C,A,↑,←,→,↓,B,C,C,C,↓,↓,A,B,C,B,A
So yes, you have to hold AB+C on the 2P while you input the 1P code. I didn't try it on Freeplay, so I don't know if it works. You probably need to turn freeplay off though, to have enough time to input the code. I just put in a credit, then did the codes at the title screen and they worked. You hear the character's voice after you successfully input their code.