Are you saying one device is only a backup. i.e. All data entry is done on one device, and all that needs to be done is that that data needs to be copied over to the other device on a regular basis (Data duplication rather than syncronisation). No additional data being kept on the second device.
If this is the case, there are a number of options.
Could just be a question of deciding which files/folders need to be backed up to MMC and then restored to the other device having placed the MMC card into the second device.
If you need to duplicate between 2 MMC cards without a PC and Card reader, it gets more complicated. You might be able to zip the content of the MMC and then copy to main memory on the second device, or use IR transfer between the two.
That might be another option for main memory as well. Zip the data files and IR transfer to other device and then unzip. Be maintaining dir structure, you should end up with the same result.
Finally, for just main memory, you could just backup the entire device, and restore it on the second device. (You can do this from the File Manager to the MMC).