Joe-
I don't know how these are formatted, but some general comment:
You don't have SD cards, you have SDXC cards. SD stops at 2GB, SDHC stops at 32GB. SDXC can hit something like 2TB although not all devices support that.
Take the original 64GB card and read the properties on your computer. Normally, SD-whatever cards DO NOT have partitions on them, they just have subdirectories (folders). Also look at the partition type, it is probably FAT32, or exFAT. If it is FAT32 and your computer is a Windows system, Windows often won't format large FAT32 devices, you'd need third party software. Similarly, if there really are two partitions on the card, you'll probably need third party software. And again, not all software will create partitions.
If you're on a Mac..or Linux....I've no idea what software will recreate the right structure on the card. But that's probably what it will come down to. Anything beyond "files on the card" and you need software that has the extended abilities to do what you want.
Odds are that someone has done that already and will soon post a comment telling you what to use, if you haven't uncovered it from this start.