+1 on moving stem.

Tandems put a different demand on the rider - harder to stand, more force on bars, etc. It's possible one of those factors is contributing to the hand numbness.

You can also move your hand a bit up on the hood since, hopefully, you aren't shifting every couple seconds. You can still reach back with your thumb to shift.

Which part of the hand is numb? There are different nerves feeding different parts of the hand, and if your pinky area is numb, that's different than, say, your thumb area. Based on affected area, you can alter your hand position a bit. I know that for me, if I put my hands down in a certain, looks-comfy way, the pinky side of my hand goes numb within a few minutes. If I angle my hand just a bit differently, I'm good for hours.

I'm sure the different shape hood was the initial cause, but a slight adjustment in hand position should fix it.

cdr