Is it ok to splice wall sheathing partway up a wall?
It's very common to need to splice wall sheathing in the open, but you usually need to float blocking to secure the joint. 

Cut a pile of 2x4 blocks at 22-1/4" (slightly less than the 22-1/2" that you'd see in a perfect world). Work along the wall and staple (or nail or screw) them in place to the existing sheathing, then continue upward. 

Rather than cutting your sheets in half, though, see if you can tilt them into place at full height. It may help to drive some 16d nails halfway in under the bottom wall plate to act as temporary supports (I don't like to see sheathing lap onto concrete--technically it shouldn't be in contact). Or hire a neighborhood kid to lend some muscle. You'll save a ton of work.