Prince can't read PDF files as input at the moment, it can only produce them as output, so you would need to use another program if you wish to concatenate multiple PDF files into one PDF file.
Alternatively, you could consider concatenating the input
files that you give to Prince, so that Prince processes one big input file and produces one PDF output file.
(If you don't want to physically concatenate the input files you could always use XInclude
to include all the files into one skeleton file and process that file with Prince).
A useful new feature in Prince might be the capability to specify multiple input files that are all formatted and place sequentially in the same PDF output file, which would remove the need for physical concatenation or the use of XInclude. Would this feature be convenient for what you are trying to do?