Here's my advice:
a. If you have a male gecko, don't add another. Males will fight and male+female will breed. If you don't know whether your gecko is male or female, you're going to have to find out first.
b. When you get the new gecko, you'll need to keep it in a separate enclosure for at least a month of quarantine. Ideally, you will have it tested for cryptosporidiosis at the end of the quarantine period
c. Assuming that you will be trying to house 2 females together, and that they are both adults, even if not exactly the same size (don't house an adult with a hatchling), remove the furniture from the enclosure, clean it, rearrange it somewhat if possible and put the geckos in together. Watch them. It's not unusual for one or both to slowly wave their tails, or for one or both to run away. If they start to bite each other, you may have to separate them.
d. Keep an eye on them over the next few weeks and months. If one starts to lose weight or look like it has bite or scratch marks, you'll have to separate them
I've generally had good experience keeping leopard geckos in small groups, but there are some that just don't get along together and you'll have to be prepared for that with a separate enclosure if necessary