I built one rifle that was "convertible" but did it as follows:
Hooked breeches (plural) - Hawken "flint" style from Pete Allan.
Two barrels. One with a drum in the plug, the other a touch hole liner.
Two locks (one flint, one percussion - Chambers Golden Age locks - there are a "few" that are drop in's in the flint/cap flavours).
You build the percussion first since support for the drum is paramount to ensuring you don't snap a drum off flush with the barrel (when an unsupported drum is repeatedly whacked with the hammer).
Then the second barrel is fitted, the lock is "swapped out" and the touch hole is drilled (and tapped if a liner is to be installed).
Inevitably the rifle is rarely "converted" - too much like work.
Better option is two rifles (or pistols) unless you want a lower cost option for two different calibers - say a 45 and 54 etc.
But the cost for the "single" gun is high - 2 barrels, 2 locks, 2 breech plugs - only a couple hundred bucks shy of a complete second gun.
Also, the hammer clearance is a little different on the lock panel between the locks, so if you go that route it's best to select a "conversion lock" (Chamber GA, L&R Classic) that is made to look like a converted flint (some of the pan/fence remains) or it leaves the lock panel looking "wrong" when you have the percussion set-up.