1. If I want to share 2 disks (eg. Quorum + Share), is there a difference between presenting 2 new LUNs to ESX vs presenting 1 larger LUN and then partitioning it in Windows?
yes the quorum drive for 2003 and 2008 needs to be its own disk, usually only present a 10GB or 5GB disk. If running 2012 its called a witness disk and can be file share or even have it locally.
2. When a disk is shared between VM's and I add a file to the disk in VM_A, it doesn't seem to show up on VM_B unless the disk is remounted. Is this expected behavior which Microsoft Clustering Service will deal with?
This is what is expected. MSCS clusters are for failover only they are not active/active only a single server can view the disk at a time. this is due to NTFS and a system locks the drive and nothing else can use it. The idea is to add the disk as a cluster drive and you can move that drive between the two servers but only one can ever use it at a time.
Cheers