Add a local monitoring template on the server with two power supplies. Edit it and add a data source of type "COMMAND". Put something like this in the command template (change the .rrd paths to your PDU component's amperage rrd):
/bin/echo print '"OK|total="',`rrdtool lastupdate /opt/zenoss/perf/Devices/10.11.12.13/PDUComponent/OUTLET1/amps_amps.rrd | /usr/bin/tail -1 | /bin/cut -d' ' -f2,2` + `rrdtool lastupdate /opt/zenoss/perf/Devices/10.99.98.97/PDUComponent/OUTLET2/amps_amps.rrd | /usr/bin/tail -1 | /bin/cut -d' ' -f2,2` | /usr/bin/bc
That should all be on one line. You may also have to adjust the paths to tail, bc, etc.
Add a data point 'total', and a graph definition for it.
The Formula Data Source Zenpack may also do the job, I haven't tried it yet. I think it's only for data sources on the same device, though.