Most secondary regulators have "high side" on each side, but it's not a high pressure as a primary regulator. The 2 ports are a "straight thru" design.
So, if your primary pushes out at say 30 PSI, you can attach the output of that to either the left side or right side of the high pressure input of the secondary.
If you mount them as a 6-gang, all 6 regulators are getting the same 30 PSI, straight thru.
Keep in mind that we're talking REGULATOR, NOT gauge.
As per this pic, all input (high side) ports are getting the same 30PSI (e.g.)