Don't cut them exactly in half lengthwise. Cut them somewhat diagonally tapered. Cut one board into two limbs, each tapering from 1" on one end to 1½" on the other end. That'll leave you with enough width for either a longbow or a recurve or a flatbow.
I'd personally go with a bamboo backing, but that's just because I don't have easy access to sinew. R/D profile of bamboo backed osage looks soooo darn good
Remember: these boards are precious and you might not get more osage boards for a while. Don't rush things that you might regret later on.