Wormtail's Wand vs. Voldemort's Wand

[grindieloe]

No: HPFGUIDX 56843

     I was listening to the graveyard scene at the end of GoF... 
We distinctly hear Voldemort say "kill the spare."  Then, Wormtail's 
voice conjures the spell.  How then does Cedric appear in the Priori 
Incantatum?  Are we to assume that Wormtail is using Voldemort's 
wand?  I am curious as to why would he do that and not use his own 
wand. Furthermore, we know from the Riddle House and after that 
Voldemort is certainly able to hold a wand even in his terrible 
state, so what is the purpose of having Wormtail use Voldemort's wand 
to perform the killing curse on Cedric? 

     Also, when exactly does Wormtail give Voldemort his wand back if 
he is using that wand instead of his own?  After he emerges from the 
cauldron, Voldie does command Pettigrew to "robe" him, but we never 
hear about him handing back a wand; yet, he certainly has his wand 
since he uses it on Harry.  

Just me rambling... :)  I would certainly enjoy hearing your thoughts 
on the topic.
grindieloe