A Yosemite install on a second partition will turn that partition into a core storage volume that can't be deleted by Disk Utility. You will need to follow
this process to kill the Yosemite partition, then you will be able to use Disk Util to expand the remaining partition normally.
I'm not sure what I did but here's the gist.
I opened Diskutil and saw that I could not delete the Yosemite partition. So I was following some steps I was reading in these forums.
In Yosemite, I ran diskutil cs list and it returned an output but I don't remember what it was. Then I ran Diskutil from Yosemite and saw I still could not remove the partition because it was the startup.
I then went into Mavericks, ran diskutil cs list and it returned an output. I don't remember what. I opened Diskutil and saw that I can now delete the Yosemite partition. But, is this safe to do? I'm assuming yet because I can't delete the Mavericks since it's now the startup.
For shiggles, I went back to Yosemite and opened Diskutil and saw I could not delete its partition because it's the startup but could delete the Mavericks one.
So back to Mavericks I go. I will keep the partitions for a while to test it out. Actually, i do kind of like it. But I also wish that Photos and some other things were included, too.