The thing I've always found hardest is finding time and opportunity to train them separately until I can trust each one to sit to the side and wait their turn. Heelwork is harder with two, as they get competitive about who is inching in front, or sulk a bit if they can't be the one next to my leg.
I've only ever added a puppy, and I do the first introduction on neutral ground, away from the house (in the garden of a dog-less friend, for example). It has worked very well (the current two are best buddies). We've had four dogs over the years, and they've all lived with two other dogs at various points in their lives, and we have never had major issues with aggression between them or anything like that - just the odd telling off.
If they get on really well, you might need to teach them how to break off play and lunacy when you want or need them to. I tell them to knock it off and a dog who ignores it twice is either put at heel for a little while if we're out, or sent to bed for a few minutes if we're home - nothing angry, just, 'That's enough, heel/bed', and led there if necessary. They learn very quickly that being a lunatic trip hazard => boredom.
I really enjoy having 2+ dogs.