I have seen many reports of individuals trying this, but being unsuccessful, and thereby coming to the conclusion that the PSP is completely limited to 32 GB of space. But I noticed that all of these tests were with the Photofast CR-5400 or CR-3100 (the latter with standard size SD cards), and thought that I would search out another adapter in case it was all down to the adapter being used.
So I bought a "Smart Dual Reader" SDA-1800 Memory Stick Duo adapter from Japan and two 32 GB SanDisk Class 4 Micro SD cards. I inserted the two cards into the adapter and upon inserting it into the PSP, was presented with a prompt to format the cards. I did so, but nothing was recognised upon completion of the format ("no card inserted"). A disappointing outcome, but I wasn't going to give up that easily.
I then proceeded to insert the cards (in the adapter) into my PC's card reader and it although it seemed to have formatted in the PSP, it only recognised 32 GB. I was a bit disheartened at this point, but then thought that I would try to format it on my PC anyway, since it was possibly just an issue with the PSP attempting to format the cards. So I decided to "quick" format to FAT32 on my PC (with "CompuApps SwissKnife V3" since Windows XP does not allow you to format volumes over 32 GB to FAT32), and saw the encouraging result of 60 GB. After this, I inserted it into my PSP and it registered the full space!
So if you would like to have 64 GB in your PSP, look out for the "SDA-1800" adapter, insert the cards and format on your computer (with the necessary software if using recent Windows versions), and then just put it back in your PSP and enjoy. Simply avoid the Photofast CR-5400 that virtually everybody using dual memory cards has. Here is a link to the one that I purchased:
I just wanted to post this so that anybody else who wanted to do it would know what they need, and not be put off by all the "doesn't work" reports. Hopefully anybody who searches the internet regarding the subject will find this topic.