Introducing another leopard gecko with my four year old female

I'm debating whether or not I can get another leopard gecko.
Right now I have a four year old female, in a twenty gallon tank, and I want to get another female.
Would it be a good idea to introduce another with her? If so, how would I introduce her? Is there any specifics I need to know? What if they fight? How would I break it up?
I know they need to be the same size and weight, and to have the new female be in her own tank for at least 60 days, but is there anything else?
Thank you in advance!

After the quarantine is over, take out and wash all the hides and things from the tank you want them to live together in. Rearrange the furniture. Put them in together and watch them. It's not unusual for one or both to slowly wave their tails. The following behavior either immediately or during the first few weeks are clues that this isn't a good idea:
--biting and scratching
--one gecko chasing another constantly
--one gecko following the other gecko around and sitting on her (this isn't cuddling, it's bullying)
--one gecko losing weight

Aliza

Anytime I put females together. I make sure to have a spray bottle ready. And a clean neutral environment, if its going to be your main tank she's in now. Clean it well while she's safe in a Tupperware or something. Clean and rearrange all decor and feeder/water dishes.
Then introduce them both in the tank at the same time. Then keep that spray bottle ready. Most females I've seen meet like this never had problems.

That's just the way I've seen used, and use myself. Others here may have another method they prefer.

I'll be doing this too in the near future (we have a reptile expo coming to Denver!) so I'll be sure to follow through with these tips! Thanks!

Spoons are also great tools for separating geckos in a tense situation. If you try to introduce them I would do so first on neutral ground and then clean and bleach the entire tank and all accessories and rearrange them so the existing female loses "ownership" of the territory (pretty much what mcwendle said). Sometimes I've seen females outright attack each other or follow each other around in an interested/aggressive manner which made me too nervous to keep them together. In the past when I introduced females I would introduce them in the morning or early afternoon, watch them for 15 minutes and then check on them every hour or two until bedtime. If no scratches or fighting are seen then I leave them alone and check on them every 12 hours or so for the next few days.

I rarely see my gals in the same hide and have had issues with fights and obesity when I was keeping more than one leo together so I've started keeping them all individually. Reduces stress for me and them!

Good luck!