The IRS rule on the basis of property held by a surviving tenant refers only to % of each tenant's contribution to the purchase price, not to their % of ownership. (IRS Pub. 551)
I agree with @Carl that it would be wise to document the contribution percentages at time of purchase.
Determining the basis is an issue only when the surviving tenant (who now owns 100%) sells the property and has to calculate his capital gain (or loss).
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
