They could be stacked. I advise against using combined exhaust ducting, they need to be separate to function properly.

Dadoes, even if the duct is larger sized to accommodate for both machines?

A larger connection pipe (at the Wye) will slow and cool the exhaust flow, causing rapid lint build up in the exhaust pipe. Use two separate pipes for safe, efficient operation.

Thanks to you both. I will have to consider my options some more as the used dryer on craigs list sold :( I was thinking of a condensing dryer, and the vent would not be needed. My area for the dryer might not hold another run of ducting. Work in progress.

good to know about the slowing down of exhaust and accumulation of lint.

I recall seeing multiple dryer venting instructions in some dryer manual. Can't find it now.

What I do and totally couldn’t be useful to you is my dryers are stacked and the vent to go outside is a few inches from the bottom dryer and couple feet from the top dryer. Husband made another dryer vent (drilled a big enough hole through the house since the current vent is on a outside wall) now I have two dryer vents one on top of each other so we don’t share a vent pipe. Total cost was 25.00 including dryer vent tube for new washer. I only have water hookups for one washer and I use a single water hose connector that splits to two and plumbed to each washer (washers are stacked too). Both drain pipes fit in current drain pipe on wall and hardly ever do they drain at the same time (stagger start times by just a few min).