Nothing, although there may be a weak point at the join between the partly cured (not dried, it is a chemical reaction, not evaporative) tow and the new application, but for one tow this is nothing.

I have never seen an epoxy where a second layer cannot be applied over a partially cured first layer so I suggest checking that statement with the supplier.  

Generally, the longer you leave it, the less likelihood there is of a chemical bond.  The other thing to watch for is bloom, which is a waxy surface which develops on epoxies, especially in cool humid conditions.  It needs to be washed off (hot water and a scotchbrite pad) before the next layer is applied.  For this reason, it is best to apply wet on wet or at a pinch wet on green (soft but not sticky).  For laminating, this should not be a problem.  For bogging over a laminate, it can be, so I usually bog the laminate in the same session as the glassing.   

Most epoxies benefit from a post cure, but a thick laminate should develop a fair bit of heat, which will speed the cure time.  Every 10C increase halves the cure/post cure time.   Be careful post curing wooden structures, they warp.

Anything you are worried about, do a little test sample.  Epoxy instructions tend to be for worst case scenarios.  The stuff is incredibly tenacious and pretty difficult (not impossible!) to stuff up.  A test piece usually removes any qualms you may have.  In this case, I would coat a piece of timber, leave it for two hours, then apply some glass.  When fully cured (put it in the oven at the glass transition temperature of the epoxy for a couple of hours when the wife is out of the house) try to peel the glass off.  

rob