Just keep on introducing them a little more each day. Always stay close by. This may go smoothly or there may be some issues.
My collie Jasmine was 4, when we brought Bella, another collie (yep I have a Bella too) home. Bella was just shy of 2. Both were intact females (not spayed).
Jasmine grew up as a pup with my other 2 older collies. So she was used to being around other dogs. One was put down when she was still a pup, the other about 6 months before we brought Bella home.
Neither had any issues at first but eventually Bella challenged Jasmine for the alpha position. This went on a few times. Jasmine kept her title. But a collie is a collie. They are not usually fighters, so just a high pitched yell and the fighting stopped. Be prepared because sometimes with certain breeds the fights can get escalated.
There weren't any issues for months. They're buds now, sleeping together, playing together. They've been together a little more than a year now. A month ago, I got Bella spayed, Jasmine is still intact. Jasmine has gone after Bella a couple times recently, barking snapping and Bella backs down instantly. I'm just always keeping an eye out. Not sure if Jasmine just wants to remind Bella she's boss. Because Bella never fights back, just wags her tail and licks Jasmine on the face.
Are your dogs fixed? That could have alot to do with it. Male and female usually get along better than the same sex. One will try to determine dominance eventually, just be ready.
Hope all goes well.
