I haven't found an official statement on this, however custom and practice, particularly at LHR, is that if you pay for a bag on a Basic fare then you, as silver, get the 32 kgs limit. The logic being that there is an extra fee for bags weighing over 23 kgs, but Sapphires and Emeralds do not pay that charge. If the trip involves a departure from the USA then it can get messy since AA have quite different baggage policies.

It may be more cost effective to split the booking, so that the Silver cardholder has the full allowance, and other travellers on Basic. This does, however, cause issues with things like seat selection. It can in theory cause problems with irrops and seat changes, but these can happen on a single booking too.