I asked this question a while back and got allot of good suggestions that I have implemented but the results are not always reliable.

My house and shop are 300' apart and are on separate electrical services. All z-wave & dual band devices in the house work reliably. In the shop, z-wave & dual band devices are proving to be unreliable.

I think since the shop is on a separate electrical service, the existing communication to the shop for my Insteon devices is RF only. Z-wave is barely being extended via two Gen 5 sirens, one at the house & the other in the shop.

I think adding a second ISY & PLM in the shop could resolve this issue but would like to verify this is possible. I have a hardwired CAT6 network cable run to the shop so LAN connection is good.

I am currently running a ISY 994i (1120) with 5.0.14 & Portal Integration

Is adding a second ISY dooable?

Yes, very do-able, but you'll have to use network resources to send commands back and forth if you want them both to respond to the same triggers.  Basically though, you just run two split systems (one ISY controls one set of devices in the home, the other controls the ones in the shop, and they can only command their own) besides the aforementioned triggers to send a few things back and forth between them (For instance, you could set variable states remotely).  Two separate sets of programs and et cetera would be involved.  Work involved to get it going, but very reliable.

Honestly I'd probably do the dual ISY setup in your case, the work would be worth it.