Did something happen that made you stress out and separate them?
The best thing for you to do is just set up somewhere for them to get to know each other on neutral territory and just take a deep breath and put them together. Here are the best tips:
"Neutral territory" means somewhere that's contained and clean and doesn't smell like either of your piggies. A bathtub is an ideal spot. I introduced my girls right after a total-overhaul cage cleaning, with literally NOTHING in the cage but the fleece lining and a bell toy hanging from the bars. You should not have any food or toys in there with them -- just open space and no distractions or hiding places.
If you're really worried that they might hurt each other, keep a couple of towels on hand when you introduce them so you can quickly throw a towel over one of them and snatch the other up.
But DO NOT DO THAT unless you literally see fur flying! The piggies will need to establish which one of them is the dominant one. With my girls, their process involved a lot of chasing and squeaking, a little bit of Kindle mounting the new girl Sonnet (which I thought was HILARIOUS -- for some reason it hadn't occurred to me that female pibs would do that too), and a lot of butt-sniffing.
Make sure you have a lot of TIME carved out to monitor this process, because this is going to take a while. You're going to be sitting there for probably like an hour or more. It's very important that you give them their space while they're getting to know each other for the first time and establishing their pecking order. Like... you can talk to them and let them know you're there, but seriously, unless they start fighting for real, just let them do their thing.
Once they've calmed down, you can move them both into the cage they'll be sharing. It should be newly, thoroughly cleaned with new lining, clean bowls, fresh water, fresh food, fresh hay. This is a good time to have some treats or fresh veggies they can share to reinforce that being together is a good, happy thing!
My last personal good tip is this: Make sure the cage they'll be sharing has TWO hidey-hut things or more, in case they don't want to share all the time. One of my girls... APPRECIATES her personal space, especially at bedtime; they would be quarreling constantly if they didn't both have their own little house to go sleep or hang out in.
