There should always be a quarantine period before putting two tortoises together. Then, because tortoises are territorial, you shouldn't add the new tortoise to the existing tortoise's territory. It is best if you can put them together in a different habitat...one that hasn't been established as one's territory yet.
You may never be able to put these tortoises together. In the wild, tortoises don't live together. They wander over great distances foraging for food and may never ever see another tortoise. But if a tortoise wanders into his territory, he will fight with it and chase it away, maybe fight to the death of one of them.
The only way I've been able to put more than one tortoise in the same pen is by keeping them outside in very large spaces with lots of visual barriers.
Separate them. Or, build a very large enclosure and put in many sight barriers, hides, food and water sources. However, they still may need to be separated after all that. Sometimes keeping tortoises in three or more, but with this you would need a very large enclosure and all the other stuff I listed above to go with it.
I agree. Any new tortoise must be kept entirely separately, in quarantine, for at least 6 months. This is to prevent either making the other sick.
In the wild, these tortoises are solitary. They roam large territories, meet up to mate and move on. They do not get lonely or need, or want, company. In captivity you cannot provide enough space indoors for them to lead the solitary lifestyle they want and they will do anything to get rid of a newcomer and have the place to themselves. Pairs rarely work evn in outdoor enclosures. Three or more may work in a very large outdoor enclosure with plenty of sight barriers, but there are no guarantees.
They must be separated NOW if you want to avoid having a sick and/or injured tortoise. This may well have to be a permanent arrangement.