I have never used TrueNAS but I guess you could provision a separate volume and use iSCSI on top of that, and serve your very large database just to make things easier.
you can in theory connect multiple hosts to the same JBOD with SAS, there are even "SAS Switches" that are built for that purpose.
But if you do this incorrectly and initiate the incorrect drives your TrueNAS data is bye-bye
The IOM6 units are just dumb expanders, they do not support zoning etc splitting of the shelf.
They just give direct access to all drives and thats it.
On paper you can split it by just connecting both and making sure you dont use the same drive for both servers.
(Would expect you to need the interposers if this is sata drives)
Have not done this myself but ive seen it done in multiple setups.
The cleaner approach imo would be a virtual truenas etc file server that runs on the host needing lowest latency and shares to the rest.
I had it setup as a virtual instance at first, and for some reason I kept running into problems - though I don't really remember what they were. I will probably end up going back to that for simplicity sake.
edit: I think the problems i had were with hosting jail services on the already virtual instance of TrueNAS. I moved away from that completely and just use it as a simple NAS now, so maybe I won't run into those problems again.