It depends what you want to do with them...
I don't think you need a Gold subscription to use Media Center, however, you do need one for many of the apps. E.G. Sky, Lovefilm, Netflix etc...
If all you are doing is streaming your own media I think you are okay, if you want the added functionality then you need Gold.
I think you also need to be signed in to play .MP4 files (something about the codec) but you can do that on a Silver membership.
Your other problem is that you can only be logged in on one Xbox at a time (using the same gamertag). It won't matter if one Xbox is on one gamertag and the other a different one but trying to log into the second xbox on the same tag as another will kick the first person off.