Generally, it is not recommended to combine Forms 1099-R.

However, if there were no taxes withheld in Box 4 and/or Boxes 12-17 and Box 2b and Box 7 are the same, then the Box 1 and Box 2a amounts may be combined from multiple Forms 1099-R.

Most of the tax treatment on Forms 1099-R is determined by the codes in Box 2b and Box 7 so those boxes must be the same.

Also, since you appear to have the TurboTax CD/Download product, you could create 3 copies of your tax returns to verify the calculations:

1. In the upper left, you could select File > Save as... and save copies of your tax returns: 2 copies with separated Forms 1099-R and 1 copy with combined Forms 1099-R.
2. For the 2 copies with separated Forms 1099-R, navigate to Forms Mode and add Form 1040 Lines 15a, 15b, 16a, 16b, and 64, separately.
   
3. Compare each total to Form 1040 Lines 15a, 15b, 16a, 16b, and 64 on the 1 copy with combined Forms 1099-R.
4. Repeat Steps 2 & 3 above for the relevant state and local tax return(s).