[Hack Work]

A GEC cant have any splices unless its an irreversible weld/crimp... in lay mans terms that means a continuous wire from panel to rode/water bond.

There's more to it.

The GEC would be the conductor going from the panel to the water pipe. But you can splice on another conductor to go from there to the rods, and that does not have to be irreversible because it's a bonding jumper and not a GEC.

The same with 2 panels, you can splice onto the GEC going into one panel with a non-irreversible splice to bring the bonding jumper into the second panel.

 

[Hack Work]

And if the water pipe is plastic? Then I can skip the GEC?

No, then the GEC would be the conductor going to the ground rods or whatever other type of electrode you were using.

 

[Hack Work]

Here's what I was talking about with 2 panels like the OP's 2 ATS's:

The GEC is the conductor across the bottom, the taps are just bonding jumpers and can be connected with non-irreversible splices.