You have not adequately described the problem. Free Dictionary defines "merge" to mean : merge. In this case, perhaps combining two flows stepwise.

This term: "evenly merge" does not have a clear definition that I know of.

Did you mean: ten rows from File1, then one row from File2?
That seems rather easy - too easy for this question - so what is it that you mean by "evenly merge".

Ooops, update: I guess it really is simple. My bad. Loop for 10 lines from File1 and send each line to File3. Send one line from File2 to File3. Loop back for the next 10 lines from File1. What's the problem?