If two people signed a car title can only one register the car ?

If two people signed the title as purchasers but only one wants to actually register it, then you should need a assignment of title (Q-1). That way the person that does not want to be on the registration can then "sell it" to the other that will be on the registration.

Depends how it's signed. You would both be on the reg unless you get a q1 and you both sign it over to just one person.