You can take data from multiple files and add it together. You can also take data for 'in' from a different file to the data for 'out'. You need to know the DS names inside your rrd files though. You can see these in the 'debug graph' output for your graph. If you get them wrong, weathermap will produce an error in the log that tells you what the possible correct ones are, too.
NODES a b
TARGET rrdfile1.rrd:dsname:- rrdfile2.rrd:-:otherdsname
So that is taking the DS named 'dsname' from rrdfile1 and stuffing it into the 'in' value of the link, then taking the 'otherdsname' DS from rrdfile2 and using that for 'out'. If you needed to, you can repeat this to add several values for each of in and out.