If you place them in corners, assuming that works well, you might also spot-treat the walls to either side of the tweeter with thick foam. The idea is to selectively target the very wide-angle off-axis energy from the tweeter down at the bottom end of its range, where its radiation pattern is the widest.
I have not tried this specific application of spot-treatment with foam because I've never been in the same situation as you, but I have used it in other situations. Don't overdo it with the foam or you'll suck the life out of the sound. I'd use a strip about 1 foot tall and maybe 6" wide, centered at tweeter height, right were any energy traveling sideways across the baffle would hit each of the walls that meet in that corner. And it doesn't have to be foam - thick textiles work fine too.
Duke