In calculating Line 126, why does Turbotax require the user to manually input 50% of the CCA amount for a 50/50 co-ownership on an investment condo, but automatically uses 50% of the CCA amount for a 50/50 partnership?
When claiming Rental CCA there are different rules for Co-Ownerships and Partnerships. The following article explains it quite well, here's an excerpt from that article.
Co-owners may then claim depreciation (Capital Cost Allowance or CCA in tax lingo) against their net rental income, individually. With a rental property, it is not possible to create or increase a loss with a claim for CCA unless the co-owner has other rental properties that show net income.
If the owners are partners, then the net rental income must be allocated to the partners according to the partnership agreement. Normally, this would be an equal share to each partner if they made equal investments. However, the agreement may split the income in any other way. The reporting of the income must match this allocation. Depreciation is claimed at the partnership level in this case, so partners are not free to choose their own level of depreciation. Any expenses paid personally by a partner may be claimed against their portion of the net rental income.