You should be able to identify each TSP from the distribution Code in Box 7 (Distributions from Regular (Traditional) TSPs and Roth TSPs use different codes).
A Regular (Traditional) TSP won't have a unique code - these accounts use the same codes as 401ks and traditional IRAs. If the account is a Roth account, the distribution code will include "Roth" in the description.
See the attached screenshot for a list of distribution codes:
There are two different types of Thrift Savings Plan . One is a Traditional and the one is a Roth plan. How do I enter it , I am withdrawing from both.
1 of the 1099R has a code 2b in line 7. So this a Roth plan?
Codes 2 and B together indicate a distribution from the designated Roth account in the TSP, not a distribution from a Roth IRA.
TurboTax unnecessarily asks if this distribution is from a Roth IRA even though the code B in box 7 unambiguously indicates that it is not a distribution from a Roth IRA. This question is nothing more than a trap for the unwary and has caused nothing but problems since answering that the distribution is from a Roth IRA will cause TurboTax to later tell you that you've made an entry error without providing any useful information as to the nature of the error.