I own 50% of rental prop., but 1099 already divides rental income in half, TT divides it again when I report that I own half

You can add the other half of the income that is not on the 1099 on the Review your Rental Summary screen:

1. Click on Start/Update next to Rental Income
2. On the screen Was Income for This Property Reported to You on Form 1099-MISC? 
3. Choose the answer Yes, some income for this property was reported on Form 1099-MISC, but I had other income as well.
4. On the next screen, click on SKIP Forms 1099-MISC
5. On the next screen, Enter Any Rental Income, you can enter the additional income in the box 2017 Rental Income