I am going to assume here you have Mavericks on the first partition, then Yosemite on the second? So when Yosemite is released you will want to install Yosemite over top of Mavericks on the first partition, then use that as your main system.
So after you have done the Yosemite install over Mavs, just delete the Yosemite core storage volume following
this. Then follow
this guide to expand the remaining partition into the empty space left by the beta Yosemite partition removal.