Hi,
I used a dual Micro SD adapter plus two 64GB Micro SD cards. I now have 128GB or storage on my PSP. I recommend this route as it's not too expensive.
64GB is SDXC which the adapters do not support. I call your bullshit, sir.
It depends on if the adapter has a hard coded limit as the only real difference between SDXC and SDHC is that SDXC uses exFAT/fat64 by default because Windows XP and later refuse to format anything above 32GB as fat32. So yes, with a well designed SDHC adapter you can use SDXC cards just fine.