Completely agree with @missbattenburg. Especially with older rescue dogs who perhaps don't have good habits trained in from puppyhood (speaking from experience, I have two rescues....)
I often regret getting two dogs, it made life far more complicated than it needed to be. I love my two dearly but aside from the behavioural reasons listed above (some of which apply in our household), there's a list of financial and logistical reasons I wouldn't recommend having two either:
For me, the cost at the groomer doubled from £30 every six - eight weeks up to £60.
Ditto with vets bills, dog training classes (should he compulsory for ALL owners IMO), worming tablets, flea treatment, food, treats, coats in winter, things like dog beds, etc. Even toys! You always have to buy two of everything. which adds up over a year.
Won't apply to everyone but renting a home. We were evicted twice (no fault of our own - both owners wanted to sell after two years). Slim pickings to find anywhere they'll accept one dog never mind two. Easier to convince landlord/owner to accept one dog than two. We've always had to put down a massive deposit for two dogs. And most landlords make us pay £100+ for pro carpet cleaners when we leave. (Thank god we own now!!)
Same with holidays (b&bs, hotels, holiday cottages etc). Can be difficult to find dog friendly holiday let, never mind anywhere that accepts two.
If you go abroad and need to put them in kennels or arrange a home boarder, dog walker etc. Price doubles! Even if you don't go abroad and need to find somebody who'll have TWO dogs while you attend a family wedding, child's graduation, hospital appt, hospital visiting sick relative etc.
The space! My mum was continually falling over her two big Labradors, their beds and other paraphernalia. Even my two little terriers take up lots of room and get under my feet. And both of us have good sized houses.
One dog is easier to walk than two. Especially if you get one that pulls like a rocket and the other is a slow plodder that likes to stop and wee on every blade of grass!!! Of course, training can sort that...
But training one dog is muchhhhhh easier than two!
We noticed our two "competing" for attention too. One will turf the other out of what he deems to be the best bed. (Comfiest or closest to us). There was also issues with sitting close to us (who can sit the closest). We have to operate a strict no dogs on the sofa policy which has been amazingly helpful with that.
Smell/mess! One easier/better than two! Washing dog bedding, muddy paw prints in autumn, wet dogs if you get caught in a rain shower on a walk, etc. Picking up poo from the garden. All just easier with one!!!
I love my two very much but I'm not sure I'd ever have two again!! The worst thing is we got them to be "company" for one another, thinking they'd play together, like each other... but nope. They're totally totally indifferent to one another. They largely ignore each other and honestly wouldn't be sad if they didn't see the other one ever again.
And god, the worst thing... when one barks the other joins in without even knowing why he's barking!!!! Just joining in for the fun of it 🙄🙄