I've made life long friends and have wonderful memories of people I've travelled with. And I also have one or two experiences where things didn't work out so well.
I think a long term DUO is difficult unless you're really lucky and you're both on the same wave length. Even then, things can get sticky.
Travelling with someone is like a marriage. It really is. It's almost a bit weird when you think about it. When you're pissed off, tired, ill, ratty and your far from home, your travel partner ends up being on the end of it. And vice versa.
If you're going to travel with someone I think you should know them first if you're making a long term commitment.
Be prepared, packed and able to travel 100% SOLO. Then if things get a bit too much you want your own space then you can separate. Even if it's just for a couple of hours, days, weeks etc. You HAVE To be prepared to go alone. It's important.
If I ride in a group I'd rather be in a 3 or 4. That way you can rotate travel buddies or go off solo without leaving anyone 'stranded'.
It's an impossible scenario to predict. If you're heading out long term with someone and your gut feeling tells you they could be a problem. DON'T DO IT... Niggles and annoyances are multiplied 100x when you're relying on each other.
A couple of years ago I did a canoe trip down the Yukon with a British guy I didn't know very well. We were alone together in the wilds of Canada for weeks on end. Just us and a canoe in the middle of nowhere.
HE DROVE ME CRAZY. He didn't get up until mid-day, smoked weed and passed out in the canoe, filled his water bottle with Whisky and got arrested for being drunk and abusive.
I could of whacked him over the head with a log and left him for the bears. No one would find him out there. I actually found myself thinking about it. THAT'S how things can get if you aren't careful about your travel company.
If the end, he ran out of money, whisky and weed and his dad paid for him to be flown out half way through the trip. Then I met a great Canadian guy in another canoe and we had a FANTASTIC time together. We're still good friends now.