Does your original therapist know that the grief counselor's services are temporary?
By splitting, it means we look at people as good or bad, like saying, oh my h is wonderful, he is only mean to me when he's drinking. We "split off" the bad part and only see the good. If we try to see 2 T's for a long period of time, we might see one as the "good cop" and the other as the "bad cop". So instead of ever resolving our feelings, we just keep running between the 2 T's, kinda like asking Dad when Mom says no.
But I don't see why your therapist would object to your seeing the grief counselor for a few sessions, since that is all it is anyway, unless she REEEEAALLLY wants you to get into a group for some reason, but it sounds like you are getting a LOT from the counselor. Like Dear Abby used to say, if it ain't broke, don't fix it!