No, they cannot combine. (in the way that you are implying).
Attacks are not made from units on the board, they are made from units "in hand". Once deployed to the board, units may not be moved or picked back up until the next turn.
Each attack is from one distinct region to another. Note that during the Ready your Troops phase, you should pick up all extra troops from each area (and can even eliminate all troops from areas if you wish) in order to make a bigger attack force.
In your example, if those troops haven't attacked this turn, then you can effectively combine them when you ready your troops.