I have a centerboard boat design, 18' on deck, that hasn't a case inside the cabin, yet draws just 10.5" of water with the board fully retracted. The case is mounted about 9" off center and is housed under furniture. There's only a few inches difference (draft) between one tack and the other, so preformance issues will be nill, plus the interior is wide open without a case in the way.
This is just one way of handling this issue and it works well. There are other methods to address this, such as a stub keel that house most if not all of the retractable appendage. With the exception of huge tidal range areas, I see no reason to accept bilge keels on a boat. Of course, as with all things, compromises have to be accepted, but I think bilge keels or twin retractables (and their related difficulties) aren't as appropriate (in most cases) as other alternatives to this set of issues.