I am running Mac OS X 10.10.5 Yosemite w/ Fusion 8.0.0 with a Windows 7 VM and having the same problem. I want to run the VM in "Full Screen" mode and extend across my laptop display and my external monitor, but Fusion refuses to do this now. Has there been any update to resolving this issue?
I had this working for a while and suddenly it stopped working.
Anyway.. here is the long list of what I did before I got it working again (not sure what fixed it but I suspect it might be because i was starting VMWare Fusion with my laptop lid closed and two external monitors hooked up)
This is what worked for me.
FYI multi-monitor wasn't working for me (two external monitors and macbook were all mirroring in Windows but not in OSX) and I closed the lid of the macbook and then windows saw the extended monitor.
Try getting your VM up first with the macbook lid up, set vmware to full screen with all monitors, and then close macbook lid