I don't see a core64.gz anymore.
It's rootfs.gz + modules64.gz you need
I am starting tinycore in a Qemu VM. Qemu doesn't appear to support launching with multiple initrds, so I had to manually combine the file like I did in my post above. I assumed that since core.gz was there that core64.gz should also be there.
FTR, originally, I thought that the gz files had to be aligned in the constructed initramfs file (thus the dd instead of catting). Turns out that is incorrect. I was able to do the following to create the core64.gz file:
cat modules64.gz rootfs.gz > core64.gz
That's simple enough that I don't practically care about having core64.gz. However, I would have expected core64.gz to exist for consistency since core.gz exists for the 32-bit stuff.