I successfully had two bitches together for nearly a decade. They were best-friends AFTER some rocky events early on that resulted in some puncture wounds and periods of crate-and-rotate and serious revision of dog management and house rules.
In my experience, you need the right combination of temperaments for it to work. Spaying has nothing to do with it - both my girls were spayed. And you have to be on your toes to stop potential problems, and be strict and fair in your response; i.e., no one is ever allowed to ignore the rules on anything that could trigger a problem.
My girls had a few fights when my youngest got to be around 1.5-2 years old and she decided to push boundaries with my other girl who was a year older. So, my younger girl often instigated if I didn't stay on her. She was just being young, pushy, and stupid. And my older girl was a bitch that wouldn't start problems but she would end them. She was a sharp girl anyway and there's more to her story than just this, but in short, I was the only person that I trusted to be able to intervene once her switch was flipped. So that also complicated things.
General house rules were:
1. They were never left out together unsupervised. Someone was *always* crated when we weren't home. And, at first, they couldn't be left out together unless I specifically was home because my SO couldn't handle them.
2. Certain groups of toys were supervision only. And by supervision, I mean I would get them out when I decided and tell each girl where they could hang out and enjoy their toys. And if one of my girls so much as looked at the other with a toy, that was it. Toys were taken away.
3. They were fed in separate rooms. High value chews were given in crates. If any body got tense about it, chews were taken away.
4. NO POSTURING. EVER. OVER ANYTHING. No staring down, no standing over, no possession of anything allowed. Everything was mine. If either one acted toward the other like they owned anything, I took it from them.
5. Growling was allowed to communicate with each other but no one was allowed to snarl. Growls were strongly enforced, by me (if the recipient tried to ignore or challenge it) so there was never any reason for things to escalate to teeth.
6. Play was heavily monitored and refereed. Early on, I would end play sessions periodically and well before anyone was worked up just to keep things from getting anywhere near problematic and keep everyone thinking clearly while being active.
Over the years, they became best-friends and I think they were able to develop that bond because of the strict rules I implemented, in addition to their temperaments being suited to it. It removed the potential for either one to control anything. Once any chance for competition was destroyed, they could relax with one another. But it was work to get there and house rules were house rules for their entire lives. No playing favorites, no letting up for good behavior. Rules were rules, period.
Both of my girls were healthy and in their prime at the time and they both put holes in each other on (thankfully, only) a few occasions until I was able to sort it out. You say that your current girl has early signs of spondylosis? I would seriously think about what position you might be putting her in if she were to end up in even one fight with another, presumably healthy, adult Dobe bitch. What might happen to her spine if she's attacked?
Not everyone's bitches fight even one time. But I know of enough who have that I wouldn't totally dismiss it as a concern, especially considering your current girl's spinal condition that greatly elevates her risk of serious injury.
ETA...Also, you realize you posted this in "Breeding and Breeders," right? If you didn't want opinions on breeding maybe this wasn't the best sub-forum...? LOL Just sayin'.
^^^^^This is fabulous!! I am living almost this exact scenario right now except that there is 6 years age difference in my two girls. I also have an old male who totally stays out of bitch issues - he is NO dummy haha! I have added training collars to my girls and they have learned to respect the audible buzz.
This is the second time I've had SSA issues with two girls. As I've never had two boys, I don't have to worry about that.
Mary Jo Ansel
AKC GRCH/UKC CH Fitzmar's Command A Minute CGC "Harvard"
Fitzmar's Victory Hop Devil RN CGC "Jezebel"
Ch Jalyn One Moment Please "Mabel"