So to sum it up you may now connect more MMC cards to same pins but they will run in legacy MMC mode (max 20Mhz) and there may be bugs since this was never widely used. I'm also not sure about power requirements of such solution (i.e. if the slot in n770 can cope with powering two cards at once).
As for SD driver it is already present in the kernel.
If I understand it correctly having support for two slots in OMAP1710 CPU is not enough. You need additional hardware for powering up and driving the slot. Such second chip is missing for the second slot and if present in must be controlled from SD/MMC driver in linux kernel (i.e. powered up and down at right time) for the slot to work. Search this forum for both n770 and n800 schematics and see how SD/MMC slots are wired.