Switching the effects between being in the FX Loop of one amp vs going to the front end of another amp can be done, but it can cause some strange problems since the level change between the two routings can be pretty big.
It would require two A/B switches... or two of the Isolated Loops (one from each HEX). One will A/B between the FX Send of the Marshall or the output of the last "front of amp" pedal. The other will A/B between selecting the FX Return on the Marshall or the Instrument Input on the Vox.
I'm getting output from the Vox, but only very faint output, barely audible, on the Marshall...probably have something connected wrong, ...the isolated loops still have to be connected to the correct IN or OUT of the chain right, I'm just not understanding why the Marshall getting sound thru... I think it's what needs to go where on specific isolated loops and where does the Marshall INPUT cable need to go?...I have all four Hex units now, and I tried hooking things up like you mentioned...so here's the setup:
left to right in the rack (this is before using you're connection info in your last post):
Hex 2 (front of amp fx, input 6 is the last front of amp effect pedal) Hex 1 (front of amp, output jumpered to Hex2)
Hex 3 (time based fx, input of 1 jumpered from Hex 2 ouput) Hex 4 (last unit...time based fx, output jumpered to Hex1 isolated output to switch between Vox and Marshall)
...before realizing the time based fx sound pretty bad into the front of the Marshall and wanting to take advantage of it's fx loop, it was pretty simple just to use Hex1's isolated ouput to switch between it and the Vox...but now since I want to use the time based fx in the Marshalls loop but still use the front-of-amp pedals in front of the Marshall at the same time, I'm getting a bit lost lol...
So I have 4 isolated loops to take advantage of, could you possibly give me a specific jack by jack account of the SPECIFIC two of the four isolated jacks and what exactly needs to go into them, to get this to work? and where does the Marshall input jack plug in at?