Here's 2 ways, but there's probably even more.
You can try just using p2stateno in the Helper's HitDef, but if memory serves me right it doesn't behave as documented (using -1 for "do nothing")
The condition statement should only trigger to send P1 to State 1234.
p2stateno = Cond((numtarget && numparent),Cond(((target,ID = parent,ID) && movehit),1234,-1),-1)
If that doesn't work as expected, then instead I think you want to try sticking a TargetState after the Helper's HitDef like so:
type = TargetState
trigger1 = numtarget && numparent
trigger1 = target,ID = parent,ID
trigger1 = movehit
value = 1234
target = 5678
... and in the HitDef you'lll need to assign a specific ID if it hits P1, for TargetState to only affect P1
id = Cond((numtarget && numparent),Cond((target,ID = parent,ID),5678,0),0)