Hi,
You can't split the volume literally, but you can achieve your goal in few steps:
1) Create new volume / datastore of the desired size, but thinly provisioned (vol guarantee set to none)
2) Migrate VMs gradually - depending on vSphere licensing either using Storage VMotion, or cold migration
3) Shrink the original volume to the desired size
4) Make the new volume thick provisioned (gurantee set to volume) - if needed.
Regards,
Radek