To combine the hives, first make sure you have two queens, one in each hive. If one queen is laying and the other is not, keep the one that is laying. If you only have one queen, skip steps 1 and 2 below.
1. Kill one of the queens.
2. Wait 12 to 24 hours.
3. Put the box containing one hive on top of the box containing the other hive. Don't mix the frames in the same box for the first 2 days.
Some people separate the two hives with newspaper and let the bees remove it. I don't think that's necessary, especially since both hives are fairly weak.
You can reduce fighting by mixing a dilute solution of vanilla extract in water. 4 or 5 drops of vanilla extract in a quart of water. Spray the mix onto the bees in both boxes. This will mask any queen pheromone scent and make it easier for the bees to mix. After 2 days you can consider it to be one hive and intermix the frames as you wish.