Maybe I'm not understanding you...Thanks for your patience
Do you want to know if with a tv tuner card in the core and not in a MD, can you watch live tv on a MD? Yes, and it's all automatic. As long as you have a tuner on any LinuxMCE box that isn't being used, you can use it for live TV on any MD.
Do you want to know if you can watch 1 channel, using just one tuner, live on multiple MDs at the same time? That's the question we've been discussing, but maybe not what you are really asking about.
I would like to know, can I watch 1 channel, using just one tuner in the Core (not in a MD), live on multiple MDs at the same time
Ok, so I/we did understand you the first time.
I think I have a solution, although not perfect and I have not tried it myself. Start MythTV go into setup->TV Settings->playback, go through the pages until you come to a page with "Show 'LiveTV' recordings when using "All Programs" filter" option and check it.
Now start watching LiveTV on any MD. This should let you go to another MD, go into media library ->watch recordings and select livetv. If I understand it correctly, anything you are watching on the first MD will show up on any other MDs that are watching the livetv recorded media. It's not going to be completely in sync, but if you FF until it's caught up it should be close.
edit: I haven't tried it, and I don't know of anyone else who has, but I think others would be interested. I can't test it myself right now, but I'm curious. Let us know if this works for you.