[CentOS] (?) Dual-monitor wallpapers on CEntOS 6

I've recently set up two workstations running CentOS 6, one with an nVidia card and the elrepo drivers and one with an ATI Radeon card with the elrepo fglrx drivers. Both work well, but one aspect of the systems works different from CEntOS 5 on those systems: I cannot get a wallpaper image to span the two monitors. I have tried both with and without xinerama and there is no difference in this regard. The specified wallpaper is displayed on both monitors.

Has anybody else run across this or a solution? I freely admit, this is a trivial issue. It's strictly an aesthetic matter and I'm curious as to why it doesn't work the same. Just wonderin'