USB Mass Storage Device automatic backup system - How many USB Host Controllers are required?
SD Cards use their own protocol, not USB. You don't need a USB host controller to talk to them. You can either use SPI (though this won't get you much data throughput) or their parallel interface mode. There are plenty of micro controllers around that support both USB HS and have an SD Card parallel interface built in (look for ones with an SDHC).

What you are proposing is not actually that hard, but it is a high speed board that you will have to design, and you'll have to put together the glue software to do the copying, including a FAT32 driver that can interpret the raw data on each storage device. Unless your time is very very cheap, you would be better off buying a single board computer that has this capability built in, then you can do everything with a one line Linux shell script.