From the Microsoft Sync Framework 2.1 SDK download page http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=23217
in the last part where it says instructions:
It is not possible have full 32-bit Sync Framework 2.1 SDK installed side-by-side with the 64-bit SDK; therefore you will have to install one version of SDK (64-bit) completely and only selected components of other version (32-bit) of SDK. See Sync Framework Tips and Troubleshooting topic for details.
But that statement isn’t totally clear: The way to install for 64bit development machine is to download the x64 for the SDK and the fill the x86 parts with the redistributables from http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=19502
It works for me at least. Hope it helps.
Unrelated tidbit: I broke the Microsoft Forums layout when I posted this as a reply to http://social.microsoft.com/Forums/en/syncdevdiscussions/thread/34a04f62-014a-48a0-9397-256b6ad1387b
Check this out: