... 
it never occurred to me that I couldn't boot both 64 & 32
from the same GRUB. 
...
and installing TC32 needs a different grub ?
Where is THAT grub ?
OK labeas, Heads up!
Yes you can boot both TC-10-x86 and  TC-10-x86_64 from Grub2 (but you must install the 64bit version of grub2)
Please don't use TC10-x86 (32bit) version of grub2, it only for old 32bit EFI firmware, specifically early and limited manufactured Apple MacBook 32bit EFI firmware.  I am not aware of any other Intel based hardware that ever used a 32bit EFI.    A PC which has a 32bit EFI firmware requiring a 32bit EFI grub2 is an extremely rare species and I doubt very much if you have one of these.  So Please don't use the grub2 32bit version on any modern 64bit hardware. 
From Grub2  "
64 bit version installed from corepure64"
   you may run any and all these TC versions
       TC-10-x86        (tinycore 32bit)
       TC-10-x86_64  (core64)
       TC-10-x86_64  (corepure64)
Didn't more experienced TC users, see that of
the several <grub.cfg entries> the 32bit one had it's files in a
different dir
The only way to boot either 32bit or 64bit TC versions is for each version to have a dedicated TCE directory,  I noticed but that is the correct way to accomplish multi boot scenario. 
In fact I run the last three 8,9 & 10 TC versions each with "tinycore 32bit", "Core64" & "Corepure64" editions from my current USB Stick(s) using grub2 (64bit)
Each version and each architecture runs from a dedicated tce directory.