The distinction is that, I think all 2 RB sets have been with Vitale as a traditional H-back/fullback.
I think there's definitely opportunity to run some 3-wide, split RB sets, with Long and JJ together. I believe that Vault has seen some time in the slot already, but Long and JJ seem less suited to that role.
I don't know that we've seen a McCall offense run split backs at all, but the cupboard has been so bare for so long - he's never faced a situation where 2 or 3 RBs might have justified playing time.
We haven't seen much productivity from the WRs so far, though Thorson's inexperience and inconsistency have been a contributor as well. He's hit one long ball that was dropped, and missed every other shot, I think. Typically, Vitale in game two and Jones in game one have been relatively safe, short completions, with Vitale out of the backfield frequently.
Splitting backs leads to some versatility, albeit in a different way than we've seen since Mick has been here. I hope the approach is considered.
Three WR doesn't really reduce the reliance on an underperforming WR group and sacrifices the formation variety having a SB on the field brings. That's why Vitale stays in for some of the two RB sets -- the defense isn't able to identify the run-heavy set and substitute accordingly, plus the SB should be a better blocked than another true RB.
Not saying we shouldn't see more of our RB group -- I think Vitale and Vault probably shouldn't ever really come off the field, moving around in the backfield and out wide -- but there are solid reasons we haven't seen two "true" RBs together on the field more often.