iMac & MacBook Both Firewired to LacIe HD's - Can't View Both HD's On Both Devices?

So here's my new issue with trying to hook-up both my (2) Lacie d2 hard drives ….. to both my iMac and new 2015 MacBook Pro (13").

I have a wiring combination of "Apple Firewire to Thunderbolt adapter coming out of the iMac > to 800/400 firewire cable going from the adapter to the Belkin hub < and Firewire 800 or 400 cables individually connected to the Belkin hub as well (See attached set-up diagram below).

I've had the above iMac to Lacie d2 drives hooked up, by firewire (through my Belkin Firewire hub), for about three years now with no problems in viewing either Lacie drives on my iMac (either seeing them individually, or together on my desktop).

So …. in attempting to also have a new firewire connection from my "thunderbolt" MacBook Retina, to the (2) Lacie d2 drives ….. I bought another Apple firewire to thunderbolt adapter, and a firewire 800/400 cable to connect to the Belkin Firewire hub.

I had expected to be able to see both Lacid d2 drives show up on both the iMac and the MacBook Retina (at the same time). BUT, have found out that either the iMac has to be shut down to see the (2) LaCie d2 drives on the MacBook desktop … OR …. the MacBook has to be shut down to see the (2) LaCie d2 drives on the iMac desktop …. but that the iMac and the MacBook can't connect to the (2) Lacie d2 drives at the same time!

I would have thought that a simple firewire connection, through the same Belkin firewire hub (that shared the Lacie d2 drives with my iMac) … would have been able to share the (2) Lacie d2's at the same time on the MacBook as well.

What am I missing here in this set-up …. or is this the best it gets … having to shut each device down to be able to see the (2) Lacie d2's on the the other device?

This is a diagram of how the set-up currently is:

Any help or assistance will be much appreciated.