I'm developing a wordpress plugin for a client. I modified two different plugins (licensed under gpl2) with similar functionality and combined them into one to match my client's needs. I'm giving my client the modified plugin under gpl2 licence. Am I required to tell him it's modified version of another plugin?
Yes, you must indicate that you modified the GPL software. Section 2a of the GPLv2 license answers your question specifically:
You may modify your copy or copies of the Program or any portion of it ... and copy and distribute such modifications or work under the terms of Section 1 above, provided that you also meet all of these conditions:
a) You must cause the modified files to carry prominent notices stating that you changed the files and the date of any change.
If it is "take 1/3 of A, 1/5 of B, mush in with a whole lot of my own", the "modified" part is quite murky. As a courtesy, I'd say something along the lines of "Parts of A and of B used" in the general README, perhaps (if identifiable pieces remain), state in the pieces (files) "Adapted from file F of A" or some such.
I'm not any kind of lawyer, just trying to apply common sense/courtesy. In the end, the idea is that if A is redone, bugs are fixed, ... the happy user of your product C can trace back where it has to be updated to match.