Definitely start him in an open bridle.
I did when I trained my 16.2 horse to drive. I did every new step in an open bridle - even just putting the harness on. He’d never been on long lines before, never worn a crupper, and never seen or felt anything behind him.
When I finally switched him ground driving in a blinder bridle, I would take it off and go back to the open bridle when I added practice shafts, and then again when I added in dragging objects.
I am glad I did that. He can turn his head around enough to see the cart and me when we’re stopped, with the driving bridle on, and he knows it’s there and what it looks like and couldn’t care less, because it was all introduced so he could see it. I also had someone pull the cart, and hand walked him behind it so he could see it.
I don’t have much experience teaching horses to drive, but I do believe it helps a ton to allow the horse to see what’s happening.
I have never heard if it is safer to drive a pair vs. a single. As others have said, there are pro’s and con’s to both. I’ve been told many a time that a 2 wheel vehicle is more likely to flip if you’re trying to take a quick, tight turn (like a marathon setting) than a 4 wheel. I’ve never driven a pair, and I’ve never driven a 4 wheel, so I can’t comment, but I have seen plenty of vehicles that are 4 wheel, driven by a single or pair, that have flipped, so I’m not totally sure it really makes that much of a difference.
I’ve heard it said that people will rely on “old faithful” to keep a “young gun” under control when driving a pair, but I think that could only get you so far.
I just started driving a tandem, which is 1 horse in front of the other. I’ve read that this is one of the hardest things to drive. I can say from personal experience that it definitely is. With the leader way out there with nothing to keep him in line, it is very hard to keep the tandem straight.
Controlling 2 mouths with 2 sets of reins is also a bit tricky. I’m still working out the kinks myself. I can’t imagine controlling 2 mouths with only 1 set of reins. For the tandem, when I need to make a curve, (say, circle to the right) I need to actually hold my wheeler left, so the arc stays smooth, while my leader curves right. So I need those 2 sets of reins to control each horse individually. But with a pair, they both turn at the same time. But still, without being able to communicate to each horse individually , I imagine driving a pair is pretty difficult. Hopefully someday I’ll get the chance to drive a pair (and a 4 wheel!) !