Oh dear, this has been discussed a zillion times.
multiboot flashes kernel onto NAND.
u-boot loads kernels from any partition.

In that moment you select a kernel from multiboot, this kernel is flashed and the u-boot attached kernel is gone for good.

You should decide to use u-boot only.
You may boot all Maemo kernels, other OSes and also nitdroid (see forum.nitdroid.com).

There is also a possibility to have u-boot combined with multiboot (like I still have, manual edit of /sbin/preinit necessary), but it is not really an option when you do not know what each of the systems does.