If you have drywall as a predecessor to taping with an SS constraint, you cannnot have an FF predecessor as well- you would be to link start date to ther start AND finish of the predecessor.

I would suggest that if you know the amount of time between when drywall should be complete and tryingtaping should be complete, you use an FF+ predecessor. If you assume that taping should finish 1 day after walling finishes, the the predecessor for taping would be (drywall) FF+1d. This would link the two and ensure that taping i salways seen as finishing after the walling.