What if Robb did the show’s thing and sends only 2000 men against Tywin to the Green Fork and keeps Roose Bolton close to him? There is no second Stark army on the field, but everyone’s with Robb after the Whispering Woods in RIverrun.

Interesting question!

I mean, there would still be a second Stark army in the field – namely, the ever-changing, ever-confusing Riverlands army. 

The main difference is that rather than having one army of Northern and Riverlander horse, one army of Northern foot, and one army seemingly made up of Schrödinger’s cats, you’d have a Northern army of horse and foot and a Riverlands army of horse and foot.