I've always had multiple dogs since my first dog was 18 months old, over 23 years ago! We're down to 3 at the moment having lost our lovely lurcher a few weeks ago and have (albeit temporarily for a few months) had as many as 9!
Training - takes three times as long as you teach each dog something, then work on it together.
Visiting places/other people - can be harder with two big dogs rather than one, but probably not significantly.
Otherwise its all the stuff you've already thought of, cost, space etc and whether they get on, there are no guarantees there.
9 months is start of adolescence for large/giant breeds and as you hopefully know, adolescence lasts probably 12 months, possibly a bit more depending on which large/giant breed.
This is a common time for people to realise 'actually, I can't do this' which is why so many adolescents end up in rescue (And why breeders may run a dog on to see how they develop for show ring/breeding prospects and then suddenly realise 'damn I am too old for this actually...' ) - its also a tricky time to intro a new dog.
It can be done though - first impressions count, and second, third and fourth ones - try to set things up so theres no opportunity for misunderstandings, argument or competition. It all needs to be light, easy, fun, calm... so probably separate them a fair bit, have one dog per person for a while and ensure they spend more time with a person than they do either alone, or with one another (separation anxiety issues are also rife during adolescence and particularly when a dog changes home!)...
If they see one another for training and relaxed calm cuddle time and for walks (that again are calm, involve lots of sniffing, some training etc, not wild whacky full on wrestling zooming walks, avoid those for now!), then you build a strong association with one another, and generally being calm, listening to the humans etc.
I love having multiple dogs, but it does take some management!