This is not an issue I have dealt with personally (the Thrift Savings Plan didn’t exist during my long-ago military days), but from what I have read there is only one significant factor which would weigh in either direction.
Advantage: If you combine, you get the convenience of only having to administer one TSP account, so there is that.
Disadvantage: On the other hand, when it comes time for TSP withdrawals, if you still have two accounts that would likely give you greater flexibility in dealing with the odd restrictions the TSP currently have imposed. Hopefully the TSP will make some changes to that in the near future so it won’t remain a consideration.
I’m sure that some other readers have looked more closely at combining military and civilian TSP accounts and may have some other thoughts to offer.