I've got a CentOS box collecting SNMP data from some networking equipment, and publishing MRTG graphs on a network status page for our company. We have two physical connections to our primary ISP connecting into two separate physical routers in our core.
I have MRTG polling both routers and getting traffic graphs dumped into two separate RRD files. I've found a few examples of MRTG configs to combine interfaces during polling, but it seems to only be possible with multiple interfaces on the same device done in the device config before the data is ever polled.
With the resulting RRD files I have, what is the best way to aggregate the two interfaces to a single file, resulting in a single sum of the transmit and receive values as it's passed for graphing?