I unscrewed the lockring, and *both* BB cups rotated - what's going on? I'm fixing up an old tandem, and the cup-and-cone bottom bracket at the front sounds very gritty, so I picked up my c-spanner and applied it to the lockring.  With a bit of effort, left cup and lockring both rotated together.  Not hugely surprising that they're stuck together - both bits are quite old and grimy.  But then I noticed that the right cup was rotating as well - as if it were rigidly attached to the left.  The entire set (two cups and lockring) won't easily rotate more that about 20 degrees,and I didn't want to push any harder.  How can this happen - aren't cups meant to be separate?  Will I damage it if I keep rotating it?  How should I unscrew it?

Some more details:
The tandem is a 1940s Claud Butler - spec here: http://www.nkilgariff.com/CBcats/Cat_40/CB15.jpg. The bracket is inside an eccentric casing.  I can't shift this at all, so I don't know how it's all connected inside.  BB make is Bayliss-Wiley.  

Photos are here: http://www.flickr.com/photos/88429902@N04/
This a request for clarification rather than an answer (sorry, I don't have enough rep to comment yet).

First thing to assess is the threading of the right cup.

I assume the BB is English threaded, because it's a CB product; if this is correct then when you unscrew the right threaded left cup, the left threaded right cup would rotate in the same direction, hence would unscrew itself. Can you see a gap developing between the frame and the right cup when you unscrew the left cup?

If my assumption were correct, the fact that they do not rotate more than 20 degrees might indicate that there's some sort of connection (intended or not) between the two. By unscrewing both sides simultaneously you are stretching the connection, hence the block at only 20 degrees.

Finally, sorry if the following comments sound offensive. Depending on your experience in bicycle repairing:


are you sure you are not rotating the whole eccentric when you rotate the cups? I can see you have an eccentric in your pictures.
you might be surprised on how much strength one needs to unscrew old BBs, so when you say "won't easily rotate", what do you mean exactly? Are you applying strength with the arm only or are you using your body weight?


Cheers