If you think about this, your clamshell switch probably has open-off/lock-closed..
So if you found the same 12v control wire, a switch could turn the solenoid valve on-off to power the A port to feed the log splitter .
So, when you want to use the clamshell, you use the reg switch.
When you want log splitter flow, us the on=off switch.
Make sure the log splitter switch is off when using the clamshell.
A toggle switch from off to on would work fine.