I think you misunderstand, you never get a double credit. You only get a credit for what you pay for. Suppose the solar system cost $10,000. If both owners paid half the cost, then the two owners can each claim a 30% credit of $5000, equals $1500. If one owner paid for the entire system, that person claims a credit for 30% of $10,000 = $3000 and the other owner claims no credit. The maximum possible credit for a $10,000 system is $3000, the only question is who paid for the system and gets the credit. The credit will never be $6000.
So with two owners, if you can reasonably say that one owner paid for the entire system, then that owner claims the credit.
