So lets say that I'm interested in the functionality that Install Profile A offers, but I'm also interested in some of the functionality that Install Profile B offers. But technically it's not possible to fire up both install profiles.
Not really possible at the moment. Install profiles are run-once, from-scratch, all-or-nothing deals.
That's now being looked at with the alternatives Features and Patterns (two competing approaches) - which try to capture the idea of just configuring certain parts of a site recipe, and exporting that for re-use and re-combining.
You can manually grab the bits from any profiles that interest you and combine them to make your own, but that's effectively just making your own. All you'd be doing is learning by example ... which is a good start, but has no long-term advantage code-wise.
Enabling the modules you want is really easy in profiles. Configuring them however, can be tricky, depending on what they are. Each is different. Which is why I use 'patterns' to save certain config recipes.