We always tour as a pair. It has advantages that two people are less likely to make a stupid mistake at the same time, if stuff goes wrong there is someone else to bounce ideas off, and you can support each other, and on a longer multi-month tour you know there will hard times, bad weather, breakdowns etc. Also you can pool kit, share stoves, pans etc, even toolkits so you each carry less. We share a tent which also saves weight.
I am fortunate though, I tour with my wife of 40 years, we know and tolerate each other's strengths and weaknesses, when one is struggling the other helps rather than getting resentful about delays etc. I don't think I have ever met anyone else I would consider doing a long tour, sharing a small tent with!