Making 2 layer board, using ground pour when I already use ground plane?
One could write a book on this topic (and few did). I can only give you a few hints as this topic is too big.

Rule 1: Current loops must be minimized. (But even this is a book for itself) 

Rule 2: Minimize common signal paths (e.g. apply star grounding). 

Rule 3: Don’t care about top planes, this is not really important. But if you use top planes, then contact them with a hell lot of visa to the bottom plane.

I will give you some additional hints to these rules:

1) Identify each current source in your design and provide a minimized loop for the return current. Even IC supply pins are high frequency current sources, that’s why you need a bypass cap as near as possible to short this current source to the other IC supply pin (Caps are a short for high frequency).

2) If you have identified a current loop of a signal trace, try to keep this current loop away from ALL other current loops, because they will harm your signal. Properly done, this will result in a star grounding concept, where each of the current loops is isolated and your GND current return paths are connected in a single point (not necesseraly a GND plane!). 

3) If you follow these rules, you might not need GND plane separation. I think GND plane separation is generally a bad thing, because it may contradict rule 2.