Why does adding a SINGLE 1099-R for ONE person on a joint return result in an increase for BOTH persons, as well as an increase in SS taxable benefits for BOTH persons?

Joint:

combined, common, communal, mutual, pooled, shared

When you elect to file a joint return with your spouse, all of your income from all sources is combined to determine a joint tax liability.

For tax purposes, you actually become a singular taxpayer.  Some items are still allocated to the specific individuals (your retirement contributions, when you make them, etc.) but for the most part you are single unit filing. 

When filing jointly many of the thresholds are raised so that they are equal to or better than those for two individual filers (though this isn't always the case) but it can sometimes be beneficial to file separately using the Married Filing Separately filing status.  

The only way to determine this in TurboTax is to prepare entirely separate returns for each of you in new TurboTax files or accounts. (For best results don't use the one you currently have your  joint return prepared in but prepare two more additional accounts to test in.)