on page 82 of the rule book it mentions that to receive +2 cr instead of +1 from the battle standard/normal standard combo that the standard bearer must be in some way fighting the enemy. but the slann is in the second rank. I take it that the battle standard on a temple guard slann is more for the rerolls?
I think a Slann is the exception to the rule. But I could be wrong, but take a look at pg.43 in the slann description. I think that, kinda of, in a elusive way, covers that.
You get the CR bonus for having a slann with BSb in the second rank of temple guard, as the rule states he can still act as normal, and normally you'd get that bonus. So it's +1 for the slanns and +1 for the tmeple guards...