I look for at least 30 inches between vehicles and from the vehicle to the nearest obstruction on the driver's side, like a wall. If the passenger side of the vehicle on the right side of the garage is not to be opened while in the garage, 12 inches from the vehicle to the wall should be sufficient.
I suspect that the 20 foot width is an outside dimension, leaving you about 1 foot less to work with on the inside. The garage should be usable, with sufficiently precise placement of the vehicles.
Doable? - yes; Comfortable? - not particularly

My two car garage has a 20 foot interior width. It is very narrow. We have a Honda pilot SUV and a BMW M3 convertible. They both fit, but it is tight and there is no real space to store anything on the sides of the garage. It is workable, but quite narrow and not ideal. I would prefer to have at least another five feet in width.

Thankfully, we have a bit of extra length - so there is room for bikes, trash cans, etc.

Having a narrow garage is funny. Our house is well over 4,000 sq ft, but - because of site conditions (some large bedrock) they built it with the smallest two car garage they could get away from. Nothing I can do about it.

I have a 20'x20' garage (interior dims). It works, but if I could have it to do over again, I'd definitely do a min 24' x 24'garage. The 20' width is just too narrow, and we have a midsize sedan and a Honda Pilot. You have to be careful to not bang the doors.

Plus, with kids, you have to really watch them to keep them from banging the doors. And there's just not enough room for storing lawn mower, trash, bikes, tools, etc.