That is a good question.

But I assume that TL-PA2010PKIT is just a kit with two TL-PA2010P.
So, you can safely upgrade the firmware of either one with the firmware of either one.

And I assume that if something is off the TP-Link utility will advise you, since you need it to upgrade the firmware.

Seeing the firmware page of both (TL-PA2010P and TL-PA2010PKIT) on my home country TP-Link page (Portugal), I see:

TL-PA2010P V1
Latest firmware TL-PA2010P_V1_140303 @03.03.2014
Changelog: Fixed a bug that IPTV may interrupt for several seconds when STB(Set Top Box) is linked to TL-PA2010P.

Other firmware: TL-PA2010P_V1_130807 @07.08.2013
Changelog: Fixed a bug that IPTV may interrupt for several seconds when STB(Set Top Box) is linked to this Powerline adapter
Observation: For TL-PA2010P V1

TL-PA2010PKIT V1
Only firmware: TL-PA2010PKIT_V1_130807 @07.08.2013
Changelog: Fixed a bug that IPTV may interrupt for several seconds when STB(Set Top Box) is linked to this Powerline adapter.
Observation: For TL-PA2010PKIT V1

My thought are:
1) The kit was created to ease the entrance of new users to Power Line technology, and is composed of two TL-PA2010P;
2) The firmware was updated on the stand alone page, but not on the kit page by mistake;
3) The problema continued, and had to be solved again.

Just confirmed the name of your device on the back of it, and confirmed if it is the V1.