Did not think of the air pressure thing.. Your going to love this.. First ball no patch... No telling where it would go.. Think I will pass on it. But I do remember reading somewhere of it.
The first ball on powder, no patch if fine. Second ball with normal sealing patch - also fine.75gr. of 2F GOEX with this 360gr. of lead - also fine, but note, more pressure, more recoil, harder more gas cutting on the vent or nipple's flash hole.
I've shot double ball loads in several guns (but not my .69) with nary any problem - extra recoil is the only result. Generally the balls struck within 3 inches at 50yards.
A .50 single lead ball load, properly placed, will kill any black bear - or grizzly. I would be testing with 100gr. to 110gr. for accuracy and adjusting the sights to hit where I wanted.
The first ball on the powder will most likely expand against the top ball due to the pressure behind and thus 'take' the rifling. It might lead the bore, which is why I suggest a VERY thin patch - like .010" to .012", just to protect the bore from leading. The rear ball should strike very close to the top ball.
I would suggest a single ball load with that 'light' powder charge will be more effective on a bear than the 2-ball load, due to the increased striking velocity causing more cavitation of the bear's tissues. This, compared to the slower moving balls, that is. I would think the double ball load might reduce velocity to barely over 1,000fps, if that fast.
You would have to chronograph this load to tell otherwise. You would have to use your own chronograph - the possibility of damaging the machine is very real.
In my opinion, you should pass on this double ball business and concentrate on a good, accurate hard hitting single ball load that you can depend on.
Good luck!