I think the thing that is causing the problem is a bad 50-68 adapter.
I'll break out the pinouts and a multimeter soon and see if that is the case and go from there.
This looks like what I have, although I can't be certain that it is wired exactly the same way: http://www.ramelectronics.net/computer-parts/scsi/scsi-adapters/scsi-adapter-adt-6850-ff/prodADT6850FF.html
I am plugging this into the HBA, connected to the middle of a wide (68) cable with terminators on each end.
Your idea with a 68pin cable connecting the drives and a high byte adapter off the controller in theory should work. i have done this before,but using active termination on both ends is ideal i think in this situation. also beware of some UW multimode terminators on cables,they claim to support ultra160/320 and SE,but i find some don't live well with SE.Plain SE rated terminators are best. check pin 26(i think!) in the cable for term power,it needs to come from somewhere,only 1 device or the controller.
You need 68-50 adapters with high byte termination. this terminates the unused lines. most cheap adapters either don't do it or they ground the lines which doesn't always work. you either get unreliable scsi,only access to the first 4gb reliably or it simply hangs.
High byte adapters cost more but are the right way to do it.just terminate the ends of the bus as you would any scsi.
The one you list specifically says it has no high byte termination. They usually say it on the adapter if they are. like this:
http://discountechnology.com/68pin-50pin-SCSI-Adapter-with-High-Byte-Termination-Mech-
