I had two girls who got along wonderfully well, but I added the second when she was 7 weeks and the older was 7 years. After a couple of days of stiff legged walking around and occasional growls, my older girl decided the pup was "hers", and they were BFFs for a lifetime after that. 
My girl/boy combination didn't go quite as smoothly. After my old girl died at 16, I had the idea my 9 year old might be lonely. When I heard of a two year old boy needing a home, I provided one. Well, my girl hated him on sight! But over a few days we convinced her to tolerate him. They got along O.K. after that, but never developed a "loving" relationship. When she passed on, we stayed with one dog, and I think he was happy with that arrangement. My girl had never liked him much, and not having to share the human attention suited him just fine.
"In order to get along with a Basenji, you have to be at least half as smart as the dog!"
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