Wii dual partition SD card WBFS partition for Homebrew

I have successfully done exactly what you have tried. I have 32GB Sandisk SDHC which
I partitioned into two parts by using the modified 

Windows 7 64x Hitachi Microdrive drivers.

Info on how to install them here.

The first part is 10GB FAT32 and I also formatted the second part into FAT32 (for starters).

Than I put some homebrew (like SDUSBLoader 1.5) into FAT32 partition and put the card into
Wii. It found nicely all my stuff on the card and I started HBC and then SDUSBLoader 1.5.
I selected SD/SDHC as a drive and it said "no wbfs, please format". I pressed A and
it showed me 10GB and 20GB partitions to select to format. I selected 20GB partition
and after very quick formatting I just started to rip discs  Worked like a charm for me.
I can also use the FAT32 part with Windows 7 normally.

I'm going to give up this "one card method" though because it's not very convenient.
1. Windows does not understand at all the partitioning -> Cannot use WBFS Manager.
2. SDUSBLoader (Channel) always proposes first "USB drive" as a drive and I have the
change it to "SD/SDHC". Small issue but anyways.
3. All (currently available) Wii Menu shortcut game Channels can start games only from HDD.

I'm planning now to use my 16GB SDHC as a "HDD" i.e. use it via USB adapter for WBFS.
Then I'll use this another 4GB card for homebrew and covers etc.

Hope this has some help for you.