There's multiple ways, depending on how close in time the snapshots have to be, the SAN hardware you have in place, etc.
If they are a few seconds apart will that be close enough? If so, you could just create a new subclient and put only those VMs in it. Make sure the number of data readers is set to high enough that it will do them close in time. If you have 10 VMs in that new subclient, then if you don't have the readers set high enough it may not snap some of them VMs until after completing the first few so that may not be close enough in time. If you only have 2 VMs they may always be much closer in time.
If that does not work for you (and if your storage array is supported), you could use IntelliSnap to make the snapshot happen at the SAN level and put all the VMs on the same datastore. The IntelliSnap backup can be set to cause all VMs on that datastore to get snapped/quiesced when as part of the SAN snapshot. The version of the VMs as they appeared when the SAN snapshot was taken will then be what is backed up.