A 2L 1.040 starter on a stirplate with 1 vial will get you 300 Billion cells after 8 days at 28C. That would be more than enough for the 1.050 beer. 2 vials into a 5L starter would produce plenty of yeast for both beers. I usually grow more than enough and only pitch what's needed after a cell count.
Never use a stirplate with brett! Too much oxygen will cause brett to produce acedic acid which will make your beer taste like vinegar. Just let the brett in a flask (covered in tin foil) for ~2 weeks.