Having two boxers is a lot of fun, but along with the fun comes work.
We waited until Hanna was fully obedience trained as well as housebroken to get Kash. She was about 1.3 years old. It worked out well but since having him her training as regressed a bit, so now we are starting to train him, as well as brush up on her manners.
I think if you have the resources ($$) time and patients it came be done without pulling out too much hair. if you are questioning any aspect of it, then maybe right now is the time to start to plan, budget and work on training your first dog, then you can introduce the second. Also rescue may require more attention depending on their situation so maybe getting settled in the new place, is best then maybe add. Also stress on moving can also disrupt your current dog, and you want to be able to give 100% attention to any an all adjustments first - thats my opinion. Also with two it may take two to work thru the introduction. I needed my husband and breeder for the first week!!
Good Luck and congrats on selecting the avenue of a rescue!!