Do you want this tone for your audience, or for yourself on stage?
It would be WAY easier to run two channels to the board to pull this off. No need for two amps and complicated equipment.
I run two channels quite a bit. I'll run a fairly bright tone from my bass (passive with the tone knob dimed). Then I run something more "beefy" from my amp. I run in-ears sometimes and blend them in my ears. I have also run my bright sound to my monitor wedge speaker and had my amp behind me.
Splitting powered signals is going to cause you to bring a metric buttload of stuff with you to the gig. Splitting at the pre level is easy as pie.
Never mind that sending two different signals to the same exact speakers (top and bottom of your SWR cab) will likely not yield an incredible results.
Whatever you heard (if it was even a split signal to begin with) at the concert didn't happen at the SVT. I might have happened at the board. But no way did he split the powered signal from the SVT into top and bottom sections of his fridge. It's not impossible. But it's incredibly unlikely. What you likely heard was his signal from the board through the PA and his on-stage rig at the same time. HE likely never heard the split signal unless he was wearing in-ears.