Hello;
I think one 3 group machine will do the job for 2 windows.
there is a lot of busy coffee shops with only 2 group machine and I'm one of them.
good luck boss.

Sounds like your opening a new drive through and having customers on each side of the building. You might want to consider having both people trained on the machine and only one preping drinks at a time ant the other working brewed and or pastries they can move between the windows to take cash because if you have them share the machine tells me the building isn't very wide. you can always have the second steaming but the cost of grinders can make you think twice about that. also make sure that what ever you decide the boiler is big enough to handle large amounts of steaming. I use a three group head machine with a good sized boiler to be able to steam large amounts of mil during rushes. I like the three group head espresso machine for the added ability to make 6 shots at once which we do quite often

I own a two group machine, and have rarely needed more, and I'd consider us a mid to high traffic shop. I've never seen more than a 3 group machine "in action". You are probably better off with a 2 - 2 group machines or a 3 at the most. The steaming is what's going to take the time, not the espresso shot, so giving them only one wand each doesn't gain you any time, in my humble opinion. You don't mention if you're a start up or just adding a drive thru. If you're a start up, I'd for sure save your funds and stick with a smaller machine at first. It's always easier to add capacity rather than pay for (and possibly sell at a loss) capacity you don't need. Two people working four heads can get awfully crazy. Another reason to split them up by utilizing two identical machines