I wouldn't expect any improvement unless your virtual disks were on physically separate media/spindles on the host system -- Having two virtual disks in RAID0 that are mapped to one physical disk on the host will probably just slow things down as the physical drive will have to seek back and forth between the virtual disk images in order to accommodate the striping.
Even with physically separated underlying datastores I think the VM overhead will trump any performance gain.
Typically I don't build RAIDs in my VM systems -- The underlying host has RAID for redundancy (with the RAID level (1/5/6) chosen based on the performance I need from that VM environment), and the individual VMs just ride on top of that with what they see as one single big (virtual) disk.
Your mileage may vary, and there are probably good cases for presenting two virtual disks to some VMs, but I'm pretty confident in saying that performance isn't one of them.