Most of my stuff is kept in sync through a hodgepodge of online services (Google Drive, Dropbox, etc) so the files and data sync issue isn't too bad for me.As far as I can tell this should work as long as you do not have bootcamp and you have enough space for all your stuff twice with enough space left to not slow everything down too much.
Do you not have an external hard drive to use SuperDuper to back up your system and use Yosemite normally on your main partition? If you use the two partitions the one you use mainly will become much newer in terms of files and data you have made that the other will become unusable or very hard to go back to without making all your changes twice, once on each partition as it will be out of date? I am only suggesting this as it sounds to me like you will use Yosemite as your main computer with the Mavericks partition as a backup in case of any problems? Of course if you have plenty of space for two partitions then your solution would work well and you will presumable delete the Mavericks partition when the public release comes out?
I do have an external drive, but the reason I don't want to install Yosemite over the entire drive is that If for some reason a new beta release breaks one of my programs, I know they all work from Mavericks so it's pretty much instant recovery to work on that specific program. Plus I don't suffer the performance penalty from booting from an external hard drive backup. There's no Boot Camp on this computer, I abandoned Windows a long time ago.
As for storage I have 500GB and have divided it into 2 250GB partitions. This should be more than enough considering most of my stuff is in the could anyways.
On deleting the Mavericks partition when the final release drops: I'll keep it for a while just to ensure that everything works, even with the final release. It may still be a bit rocky considering that this is a pretty significant release, even after all these betas. Then after I can tell everything is fine and the first few updates to address significant bugs have rolled in, I'll delete it.