I repeat:

Do you intend to use both of the 600N radios at once ?

I think that we are all trying to understand the importance of N to
you if you don't want to use 2.4 Ghz band ?

And if you intend to use A, then there are countless adapters.

Unless, that is, you do intend to run both radios at once?

I have a client that fixes on optimization of a certain concept (in
this case, using all possible wifi bands), often without knowing why
or if he will really want/need to do all the things he insists he must
be able to do.

No offense intended, just a reality check. Once you achieve the holy
grail, will you really use both bands? I believe that you will need
two cards to do both bands simultaneously in any case.