Attach disk from multiple backends

i am new to ONE. Currently, our ONE have two storage backends for system/image DS which are ceph and local file system.
When start new VM, if we choose image from ceph, VM will have disk from CEPH, if we choose image from default file system, VM will have disk from local file system (in compute node). However when we add second disk to VM, CEPH is always used.
How can we choose datastore when adding second disk?
Thank you.

i found the way to workaround this issue.
we can tag some features for each DS, for example, local file system DS will have SPEED=10 and STABILITY=5 while ceph DS will have SPEED=5 and STABILITY=10. Then we can apply policy to the template which will choose DS based on features we just defined. SCHED_DS_RANK = “SPEED” will choose DS with higher SPEED.
However we still looking for easier solution which just add option to choose which datastore to use when add new disk.