Hey Nesto,
Another great question. I am using Brett L on mine, and will be pitching Roeselare next Monday after NHC. I unfortunately don't know exactly what blend he uses if he does use one - he just told us "Brett" when he emailed the recipe over. "We add 1 million cells of Brett per ml beer to the beer. I assume you'll figure this to be say one vile per X gallons of beer...We do not purge the barrel with Co2, but, it might not hurt. We leave a minimum of 5 gallons of head space and usually more like 8 gallons. There is a lot of residual sugar and sugar from the currants that the Brett will ferment out."
1 Gallon = 3,785mL, but I don't know how many cells are in on vial of White Labs/packet of Wyeast (White Labs has 35mL in their Brett, Wyeast has 100mL in their Brett)
He also mentions adding more Brett when he adds his bugs after the original addition.
I think the split of Brett B and Brett L is a good idea, not only to see the differences, but to blend later. You could find a good blend (50/50, 30/70, etc), or find that you like them individually.
Cheers!