LMCE does all the streaming for you. So, if you have a core in and two MDs connected to it. All MDs can make use of the tuner within the core (It is best to have the tuner cards in the core, if you stick with MythTV you can also put them into a MD).
In my setup, I have a Hauppauge PVR-500 (double tuner) and a Hauppauge PVR-150 (single tuner) in the core, so I have three tuners available in the whole house. If a MD requests a channel LMCE automagically decides which tuner will be used and does all the streaming. For my setup this means I have three tuner and they are available on every MD or for Recordings. If you have configured your cards properly, this is a cakewalk.
To avoid any problems on your side, best is to go with a Haupauge PVR 150/250/500, they are Plug'n'Play and are really easy to setup. Have a look at the wiki for some more information (supported Hardware, MythTV configuration, etc.)http://www.mythtv.org/wiki/index.php/Category:Video_capture_cardshttp://wiki.linuxmce.org/index.php/MythTV