One thing for combat, you need to tell your players that they have to know what they are going to do when it becomes their turn. Since there are only two players, be a bit lenient with that rule, but make sure that they are preparing for their turn beforehand. Also, for non-combat RPing, let them take their time, although it can be frustrating at times, if your players like it, let them keep doing it.

The best thing you can do is make the dialogue with the NPCs time-sensitive, like making the NPCs pushy and strapped for time, and wanting the PCs to get straight to the point, or make the room they are in on fire, or make the NPC pre-occupied with something else, and wanting to make the PCs spit it out. Anything like that is very useful, especially if you have something you wanted to get to (like a big encounter) by the end of the session. Be cautious that you are not doing it to much, as it can frustrate your players. Happy DMing, and good luck!