this is another topic really & my bottom line is that there is no set rule, smile.
the waiting until 2 yrs or so, I think is so that the first dog will be bonded to you.
I would say that all of ours are very bonded to both me & husband. Even the last rescue. cleome was in a gas station parking lot, nearly comatose with a skull fracture. She is about 3 yo, mix? chic/rat terrier & no clue about her previous life.
She spends her days with a 12 yo F min pin, a 3 yo male mix corgi & a 2 yo mix Italian greyhound F. They are really affectionate & play like puppies. That suprised me.
There is a poster on the Shiba forums that has a few mo old mix & just adopted a bit older mix, from a shelter both males & they are best buddies, check out the photos. Its Jasper & Bear (if I recall)
Some dogs just seem to click, but I do have years of experience (training, etc) & stay at home & we have lots of fenced areas, they all are inside family members too.