Companion ticket availability question

Hi,
I have a question about BA Companion tickets that you can get through the BA Amex card although I suspect the same question could also apply to VS companion tickets.
Does there need to be 2 "reward" seats available in order to use the companion ticket or do they come from a separate bucket.
Its hard enough often finding 1 seat available on many routes let alone 2 so I just wanted to check if these were actually of any use.
Also, can you use them on all classes or are they specific to Econ/PE?
I have a question about BA Companion tickets that you can get through the BA Amex card although I suspect the same question could also apply to VS companion tickets.
Does there need to be 2 "reward" seats available in order to use the companion ticket or do they come from a separate bucket.
Its hard enough often finding 1 seat available on many routes let alone 2 so I just wanted to check if these were actually of any use.
Also, can you use them on all classes or are they specific to Econ/PE?