They don't need both of them - either one or the other, having them both is a waste of money.
You could argue they don't need Mi-28s and Ka-52s... but they have both and they are different enough to be complimentary.
I would say the same with the Su-33 and MiG-29K2... certainly they don't need both but both have been developed and the latter paid for already.
It might make sense if those Su-33s were modernized for assignment to non-carrier duties.
The US Navy had the F-35 and the F-18... on paper they are both fighter bombers...
the Su-33 is larger and heavier and would have better range and with its main wing and horizontal tail surfaces all folding as well as its nose it can actually fit in similar sized places that a MiG-29K can fit so the hangar space argument is not as significant as some might make it.
The real difference is that the MiG-29K is fully multirole, while the Su-33 is an Su-27 that can carry Kh-31s and Kh-35s.
The MiG-29K also has fewer hardpoints, is not compatible with the Yakhont and possibly the supersonic Kalibr-variants and yes - it doesn't have canards and thus has a disadvantage in turn-radius and manuevers.
Su-33 can't carry Yakhont either... AFAIK the only Flanker that has been seen firing Yakhont is Su-34.
Both the Su-33 and MiG-29K are limited to 8gs because of their folding wings.
The only advantage, of the MiG-29KR, over the current Sea Flanker was, that it is a multirole fighter and nothing more!
So the fact that the MiG-29KR is a better fighter and a better bomber means nothing?
the MiG-29KR should be compatible with the R-37M with a 280km range...
My only hope is that they still plan to go ahead with MiG-29K introduction, but want to modernize and keep around a few Su-33s in the carrier air-wing for the long-range missions. I guess such an idea has its value too.
So pretty much what they are doing is what the Air Force did when it upgraded some bog standard Su-27SM aircraft to keep up numbers even though they will get PAK FA sooner than the navy will...
Well they can buy more MiG-29K's, newer structure, newer technology and surely can last longer.
An upgrade will always be cheaper than new builds, plus they already have plenty of operational experience with Flankers.
If such heavy fighters are needed however.. Ru Navy might might better off with restarting Su-33 production line.
I suspect they primarily want compatibility with the R-77 and other newer weapons... the Su-33 is not able to carry most of the new weapons deployed by the Air Force and Navy... it is basically a bog standard Su-27 with R-27 and R-73 for air to air and dumb bombs and dumb rockets and cannon, plus Kh-31 and Kh-35 for air to ground. I suspect an upgrade to the radar to actually detect and track sea surface targets and ground targets and to use laser and TV guided air to ground weaponry is the priority... plus digitalising all the systems will make them easier and cheaper to maintain.
This wont be a naval Su-35 we are talking about... it will most likely just be an Su-27SM upgrade we are talking about.