Phil is exactly right here...and I'm also totally confused as to what your problem is. When you add a volume pedal to the beginning of your signal chain, the default action of the Helix is to assign that to Exp2. So there's no footswitch to assign to turn on the volume pedal...the built-in expression pedal is already acting as your volume pedal. When you add a Wah to your chain, it will be turned off by default and will be assigned to Exp1 (again by default) so you don't need to do anything to get this setup to work.
So, when you want to engage the Wah, just stomp on the toe of the Volume pedal (Exp2) and the toe switch will engage and change the expression pedal to act as the Wah (Exp1). When you're done using the Wah, stomp on the toe again to switch back to Exp1 mode and the expression pedal is once again your Volume pedal.
This is the default way of using the expression pedal in Helix and I can't understand why you'd ever want to assign volume and expression to another switch anyway. If you're using the expression pedal for something else (like changing a delay mix ratio or something) then I'd suggest getting an external pedal for that and leave the built-in pedal for the Vol/Wah functionality.
LeeandTanja's post in Unnecessary tap dancing to get to wah during solo was marked as the answer