I'm a single member and owner of (2) LLC businesses. Should I file both LLC's at the same time on one tax return even if one is making money and the other is not?

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Both of your LLCs, being single-member LLCs, are disregarded for federal income tax purposes so you will report on Schedule C.

For accounting purposes, you are probably better off separating the two businesses and filing two Schedules C. On the other hand, there is no absolute requirement that you do so.

However, you will have to file two separate Schedules C if you pay employees for each of the LLCs under different EINs.