Two separate completely set up enclosures...I would not introduce for at least a year....some of course do different times. Also, if you are getting a second tortoise to create a pair---you should have a plan to maintain two separate enclosures anyways due to pairs rarely live in harmony....
also...the reason you want a quarantine is of course firstly because of health issues...but secondary reason is that you need a while to get to know your new tortoise--I mean, how would you ever know what that new tortoise normal behavior is if you don't have time to get to observe and know it...then if there is an issue when you do allow the two torts together you will be able to pick up right away if something is not going right...you know?
I have three Russian torts at the moment, all three of them are separate. Different enclosures inside and outside. Different soaking containers, food and water dishes. I wash my hands in between handling them as well. I work in healthcare and I kind of just treat them like patients.. Lol. Maybe overkill, but I know I will not cross contaminate anything.
You may want to make sure you are prepared for what can come of a pair....and you may want to plan that the outdoor enclosure you build can easily be tweaked to make two individual enclosures....an area that can be used to divide the enclosure....you know?
No. You keep them completely separate for the entire quarantine period. They each need completely separate areas both indoors and out. When you decide the quarantine period is over, then you can start trying to introduce them, but do it on new territory where neither of them has ever been. During quarantine you should take stool samples from each tortoise to your tortoise vet and have them checked for parasites and pathogens. Do this at least twice, and at least two weeks apart.
And by the way, pairs seldom work out. Think in terms of having at least 3 or more, unless you plan to always house them separately and only introduce them for brief periods for breeding.
Many people just buy a new tortoise and throw it in with their other one. This is a huge gamble and could result in either or both dying. The quarantine period is simply a time for the new tortoise to get acclimated to your foods and environment, while at the same time allowing you to observe and check for any signs of disease or sickness. Many people recommend a long time, but in my opinion, if you are fecal testing and know what to look for, any diseases should show up within 30-60 days. Many vets will tell you that 14 days is adequate for any diseases to get a foothold and show signs. The more I value my existing tortoise, the longer I would want to wait.