are the hosts joined to domain? if yes then it should be set to sync with AD
This was way back around 5.0/5.5 before I fixed it, but I had bad issues with clock drift when I tried to use synchronize with host for DC/domain joined VMs
Synchronize guest time with host I would reserve for non-domain joined machines or ones that may not have network/domain/Internet access.
I am not sure I would want to have the host synchronize off the VM DCs running on the host either.
Best bet would be to set both the DCs and the host to sync off the same hardware NTP source. If no hardware source is available on the network, a good fallback would be NTP pool or if you are running Cisco switches I believe you can use the default gateway as an NTP source, as long as it is running off an external NTP source itself.
Because the host synchronizes its BIOS off the clock time and the VMs are running off the host timing, it can get you into a weird situation where the clocks will be off if one is trying to sync back off the other, or performing non-stop clock corrections as it drifts or ping-pongs from one source timing to another.