This question asks several different things, so I will try to address them separately.
Can I combine the 2017 contributions for both accounts towards the "retirement savings contribution credit?" Yes, but you shouldn't need to. The primary reason why this is so is that, for determining the amount of potential Savers Credit, a maximum amount of $2000 of contributions can be used toward the credit. Since each contribution was above this amount, you will get the maximum amount of Saver's Credit your income allows. The fact that you contribute to a Roth as compared to a Traditional Account does not affect this. Please see this FAQ with more information: https://ttlc.intuit.com/replies/3301263
Can I combine the 2017 TSP and IRA contributions towards my Roth IRA limit? No, and you really wouldn't want to. If you did, the amount of excess contribution would be disallowed and you would have to withdraw the excess contribution or face a penalty (neither of which is that fun to do). However, in this case, it is not necessary. A TSP is not an IRA, but rather a separate pension plan (similar to a 401K). While it is possible that your income may limit your contributions, the fact that you participate in a pension does not. This FAQ discusses this as well: https://ttlc.intuit.com/replies/4795138
In short, when you enter your contributions, TurboTax will assist you to correctly claim the correct amounts for Saver's Credit as well as Roth IRA contribution limitations. In all likelihood, your amounts will not be affected.
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