The basic thing here is what's referred to as "tracing rules". If audited, you may have to prove who paid what and show a money flow path of the person claiming the deduction. With a personal joint account, all money in the account is generally 50/50 regardless of where the money comes from. It complicates things when the joint account holders are not married to each other. But if you can show that say for example, 70% of the money that flowed into the account during the year was from you, and 30% from the other party, then claiming 70% is no problem.
Overall, I can't see the IRS auditing on this specific item. But if audited on something else (or pulled for a random audit) other things could come to light.
Basically, each of you should only claim what you can support and prove you actually paid.
