Here is a patch for osm2pgrouting to import more then one file.

How to install

1. svn checkout -r 352 osm2pgrouting
2. download the patch
3. cd into osm2pgrouting
4. patch -p0 -i append.patch
5. make

How to Use:

for the first file :
osm2pgrouting -clean

for other files (except last)
osm2pgrouting -append

for the last file
osm2pgrouting -append -finalize

How it works:

you have to split a large osm file with osmosis like this way (the boundingboxes must overlap on all sides, maybe 0.1°)
./osmosis --read-xml europe.osm --bb left=14.0 right=14.6 top=48.5 bottom=47.8 --write-xml ./1116.osm
./osmosis --read-xml europe.osm --bb left=14.5 right=15.1 top=48.5 bottom=47.8 --write-xml ./1117.osm

or use osmosis with parameter --bb completeWays="yes", with no overlapping but splitting may take longer... its not testet

you can also define the boundingboxes for osmosis in a file like explained here

this patch also adds the original osm id to the ways table, and it adds the filename of the imported file