I don't think you quite got it.
He said the recruitment is coincident ie. the process is for both.
Use whatever form you can find as the selection process lands you in a Britannia hold pool for both airlines with you putting your preferences.
I'm in the hold pool now & was sent a form asking what I want to be considered for:
1. 737 only
2. 737 or 757/767
3. 757/767 only
tick only one box.
Get your form filled in now. They have alot of ex Duo people in the hold pool but those people also have other offers on the table & Britannia might find their hold pool emptying quicker than they would hope.