You can only do stereo effects with a stereo amp or two mono amps. If you only have one power section, it's physically impossible to produce two separate outputs.
As for how to run the HALO, there's a few ways you could do it.
You could try doing it like you mentioned, but as has already been stated, if you do what you're talking about in the first post, you'll have ground issues. You could resolve those ground issues with iso transformers though. Just plug an iso transformer between both preamps and the HALO, and then another set of iso transformers between the HALO and poweramps, so four isolation instances total. However, most iso transformer boxes you can buy have two sets of inputs and outputs for two signal lines, so you'd only need to buy two of those "two line" iso transformer boxes. They're pretty cheap, too. Don't run your guitar directly into them though because they kill the tone of high impedance instrument lines. But they have no effect on tone when run with lower impedance lines like effects loops and line level stuff.
The simplest way to do stereo delay would be to only use only one amp's preamp section, then run the effects send of that preamp to the HALO's input, and the stereo outs from the HALO to the effects returns of both amps. This would work perfectly well.
FYI: The only controls that would function on the slave EVH amp you have would be the Presence and Resonance controls. I believe the Volume control on the main amp would control the loudness on both amps. This is because the poweramps on EVH amps always run at full volume, and the volume controls themselves are just channel volume controls.
If you want to get super fancy, you could use a loop switcher to wire up the preamps of both of your amps as if they were individual effects pedals themselves, then route the loop switcher to the HALO, and the HALO out to each effects return jack. That would allow you to use each channel of both of your amps as you wanted. Again though, if you do this, you'll want to use plenty of buffers and iso transformers in your signal chain because it will be a ground loop nightmare without those things.