I haven't flown a DH8 for about 6 years but a vague recollection says that after the first ship's battery start, the charge rate on the battery must be below a certain value before the second engine start is commenced.
Bringing the prop out of feather brings the AC generator on-line. I think having the AC gen on-line allows more of the output of the DC gen to recharge the batteries, whilst the AC gen powers the rest of the ship's services.