My wife and I had Required Minimum Distributions in 2018. The system is not allowing me enter my wife's RMD.

After one Form 1099-R has been completed the program will take you to the Your 1099-R Entries screen.  Click on +Add Another 1099-R.

On the same screen for selecting the type of Form 1099-R that is being reported are selections for indicating who the form belongs to when filing a joint tax return.  See screenshot

To enter a form 1099-R -

• Click on Federal Taxes (Personal using Home and Business)
• Click on Wages and Income (Personal Income using Home and Business)
• Click on I'll choose what I work on (if shown)
• Scroll down to Retirement Plans and Social Security
• On IRA, 401(k), Pension Plan Withdrawals (1099-R), click the start or update button

Or enter 1099-r in the Search box located in the upper right of the program screen.  Click on Jump to 1099-R