I encountered this same situation last month and luckily, it turned out well for me (after some excellent advice from Sandy in a PM).
The first point to consider, and the most important point, is if things do go wrong and the Tortoises do end up fighting; do you have provisions to keep the new Tortoise in another enclosure? If you do and are happy with the possibility that you might have to accommodate two enclosures then go for it. If they do like each other then GREAT if they don't, at least you aren't stuck with another tortoise with no-where to place him. Sorry if this sounds extremely pessimistic but at the end of the day, we are dealing with live animals!
Secondly, is the current enclosure big enough to accommodate two Tortoises for the foreseeable future? My current enclosure for both of mine is 2 foot by 4 foot and it should suffice for at least the next two years (when I envisage an upgrade). Even still, at times I feel it is a tad too small to house them over the winter months (obviously, when summer hits, space isn't such an issue in the garden). So, the bigger the better; also, from experience, the bigger the less issues that being "cramped" will cause.
Thirdly, food costs - Remember, that the amount of weeds you must grow/gather will be doubled. If you are having trouble finding enough now, then it will be much harder getting enough for another mouth!
Fourthly, quarantine - Alot of people suggest that the two tortoises should be separated for a number of months before being "integrated" together. This is to make sure that if one Tortoise has something icky, then the other won't contract it. However, I personally skipped this stage and haven't had any bad effects (touch wood). Even still, I wouldn't recommend my approach since I was just incredibly luckily!
Fifthly, Gender - Having two males seems (from what I read) to be the most likely candidate for tortoise aggression. So, having either a male/female combination or female/female will be best. However, with the male/female remember that down the line in the future, you will have to deal with other Tortoise "shenanigans!".
Finally, if you are able to provide all the things above for the new tortoise then I say GO FOR IT. Remember, with another Tortoise, your enjoyment is not doubled but tripled. Since the two Tortoises will not only each interact with the environment but also with each other (In my case, with some very bizarre incidents). Mine haven't had any fights or aggression apart from the one isolated leg biting incident two weeks ago (I've put it down to the fact that Boadicea was sitting on the food whilst Spartacus was eating it). So, for me it was a great success, however, since Tortoises are such idiosyncratic creatures, anything could happen. So preparation for all eventualities is the key!