We went from 1 to 2 to 3 to 6 to 8. Each transition had its own challenges, but it all worked out in the end. I wouldn't hesitate to go from 1 to 2 again...the 6 or 8...well
We're gettin' too old for that many, I think!
The main thing to remember is that both dogs will still need their one-on-one time with mom and dad and that until they learn each other's dispositions, they'll need some close supervision. If they could meet first on neutral ground, it might go a long way toward letting you know if the two are compatible.