I have a phone line connected on ADLS upstairs and a second socket on the same line/number downstairs also. My question is, can I be connected on both lines at the same time? Is this a) possibe and b) allowed?
Well, put it like this: if you had ordinary phones, one on each socket, would you be able to use them at the same time (for making independent external calls)?
The same is true of ADSL equipment: it doesn't matter which socket the equipment is connected to, but you can only use one item of equipment on the line at any given time.
The one new thing about ADSL-enabled lines is that you can use ADSL equipment at the same time as conventional phone equipment, but just one item of each. So, if you had a conventional modem on your downstairs socket you could use that at the same time as your ADSL modem.
Regarding the "Is it allowed?" question -- allowed by whom? By BT: yes. By Force9 if attempting to "dial in" on a single account? I'm not sure, possibly not. But if you had a second account, you could use ADSL on your first acount, and dial-up on the second one.