Take it easy Johnny! Who pissed in your cornflakes today?
While I give Johnny a little crap for his approach, I do agree that you should not throw away 100 years of ingenuity and experience. Check out what others have done first.
I'd definitely go with the quinto-tumba combo to start. It is the traditional way to go. It also gives you the widest range of sounds and pitches of the three possible two drum combos.
You're right, a lot of drum stores package quinto-conga and conga-tumba combos, but no quinto-tumba combos. There are two reasons for this. First, your average guitar center drum shop manager know's little about the history, tuning, good use, etc. of congas. Second, if they sell you a quinto-conga or a conga-tumba set, you're immediately going to want the third drum to fill out the range. BUSINESS!!!
Zach