LinuxMCE consists of four parts: the dcerouter (core), media director(s) (where you play media), orbiters (remote controls), shared media (via CIFS, samba, NFS). The core is intended to be your main router in your home. It is recommended to have two ethernet ports, one to the internet, one to you home LAN. The core really wants to be the DHCP server for your LAN (that's how it identifies when MDs, orbiters and shares connected to your LAN).
If you take advantage of this architecture and have other machines on your LAN, then dual booting the core is probably not what you want to be doing.
LinuxMCE also is about home automation. You can control your lights, security system, VOIP system, air conditioner, sprinklers, etc. If you take advantage of these features then you probably shouldn't be dual booting.
On the partition question, LinuxMCE wants an entire drive (particularly the DVD installer). If you still want to dual boot, then I'd suggest separate drives.
From the sound of your post, you may be happier using a mythtv distro.