My advice would be to put Ruby first and foremost, are you happy with her training standard, does she need more work? If the answer's No then I'd say you would cope with a second dog, if the answer is that she needs more work then you need to ask yourself could you cope with training two dogs, regardless of the 2nd dog's temperament or training level whatever dog you take on will need some sort of training, even if it is just getting used to your routine and commands.
If you take a second dog on and Ruby isn't up to scratch, could the new dog teach her bad habits.
I'm just saying all this to give you stuff to think about, I have no ieda what Ruby is like and I most certainly have no idea of what time etc you have in your life to be able to train two dogs.
Having said all this I'm sure Robert is close enough for him to be able to help you with advice if needed.
The other thing to consider is if your OH isn't 100% committed it's probably not a good idea.
I thought long and hard before taking on a second dog , it had taken me two years to get Rufus up to scratch, he was hard work, so I then opted for an older much calmer dog which meant I still had time to keep up Ruf's work.
On that note it would all depend on the individual dog as well, if you were thinking a new pup, I would expect that to be a lot of hard work, if you were thinking rescue then the rescue would work with you to find you a suitable dog so that may be the answer.