Presuming you are starting with Airplane Private. If you go Glider Commercial- Airplane Commercial you will need to take both written tests. If you get the Airplane Commercial certificate(written + check ride) first you will not need to take the Glider Commercial written just the add-on check ride.. Avoiding an extra written test probably isn't worth delaying the Glider Commercial. You could add-on Private Glider, take the Commercial Airplane written and check ride then take a Glider Commercial check ride, that swaps a written test for an extra check ride. I'd rather take both written tests. The absolute minimum test hoop jumping would be to fly gliders with a solo endorsement until you have Airplane Commercial then add-on glider commercial. Some would frown on that, depends on your goals. Not sure where this info lives might be in the relevant Practical Test Standards, now I'm going to have to go looking...