If you have two similar voltage packs then connecting the two positves ( red) and two negatives ( black) together and then to the load ( Esc / controller) should give you the same voltage but the combined amperage rating.
It is suggested that isn't advisable to do it though, you can get a situation where one pack is trying to charge the other if you have a weak cell that the resistance is low on.
Are you trying to get increased runtime?
Similar isn't quite good enough - they both need to be the same and at the same state of charge. If there is a dud high resistance cell in one of them, it will just mean, most of the time, that that battery will be ballast. If both batteries are good, but one less charged than the other, they will try to even out. They will do this rapidly, with quite a bit of energy dissipation on the way.
The action unit has diodes to stop this, but this in turn means that the better of the cells will carry all of the load until it gets down to the level of its companion, which will then join in.