For B2B, it generally makes sense to have one person whose job it is find and talk with customers while the other can focus on building/iterating the product. It helps to be technical for this but they have to actually like talking to people and ideally have a decent network for what you're selling. This isn't exactly sales but it's closer to 'sales' than it is engineering and so generally benefits from that background/skillset.

So in theory it makes more sense to team up with someone with that skillset than partner with another engineer but then one of you has to give up on the thing you're good at to do founding sales which you may-or-may-not be inclined to do.