Firstly - do it with care.
This is at high pressure, but be aware of the pressure at each stage.
The tank pressure will be up to 3000 or 4500 psi, the output regulator pressure in the region of 800psi unless it's LP.
Running the regulator output of one into the full nipple of the other will give you around 800 psi, but the input nipple will begin at the full pressure of the cylinder - 3000 or 4500 psi.
A bit of a dodgy rig there. Best you can hope for is the main cylinder feeds your marker until the pressure drops to around 800, at what point the regulated output of the other cylinder equalises and feeds the main cylinder.
But the worry would be how the regulator and feed line will cope with psi in the thousands
Better solution is to have both cylinders outputting as if they were feeding a remote line each, then have a t junction taking both outputs into one remote line to the marker
