Well you sort of answered your own question. Pitch the lacto keep it warm then pitch the brett. People have been getting, what seem to be, weird attenuations with just lacto. It makes me want to pick up a tube and grab a differential media plate to see if its white labs putting yeast in the tube or if it's just lacto. Anyhow the decrease in pH from the lacto will help the brett with getting going and possibly provide a medium for better flavors.