witness communication when both 2 nimbles are sharing same l2-subnet

[Remon1983]

Situation:

we've got a setup where  2 nimbles hf20 are in different datacenters on one stretched l2 vlan.

Between there is a double Dark Fiber link wich is laying in the same housing, so when both fibers are down then they are separated. 

The quorum witnes server is in another dactacenter wich is connected by mpls and in between there is an ha-srx cluster wich is serving as default gateway for the nimbles to connect to the witness. 

The question is now how i can facilitate the connection to the witness server when both of the  Dark Fiber are down .

For my correct understanding it is that :

Both sides need to have a connection to the witness server in these cases but i can't think of a way to create this other then route traffic for primary management ip + diag ports on Router 1 and traffic for secondary management IP + diag ports on Router 2.

Or is  it enough so that from both sides only the diag ports can connect to the witness server  because currently traffic is flowing from the diag ports to the witness server ?

Support is giving me no clear answer and am thinking the only other way is to arrange an separate circuit as failover for dark fiber failover.