I would not be so categorical. Depends on your problem.
Suppose that you have 3 files A, B and C with data for filepile. You could concatenate them into filepile.txt which would be seen by weewx as one file.
Under unix (linux etc) : cat A B C > filepile.txt
You could also build filepile.txt in 3 steps. Assume 3 procedure p, q and r producing the 3 sets of data.
Then, assuming that filepile.txt is empty, p >> filepile.txt q >> filepile.txt and r >> filepile.txt will produce the single filepile.txt with all your data.
Hope this can help you to solve your problem. Sheers, Paul