Be aware of the following, witnesses not only are asked to complete a blood card but also to fill in an advanced medical directive which is more detailed and specifies end of life decisions. These are drafted as legal documents and I am unsure how much force they would have if your wife was unconcious and you took medical decisions on her behalf.
People who are likely to support the witnesses stand on blood are asked to act as witnesses and may be named on the card as emergency contacts.
A copy of the documents are often held by the congregation secretary and can be produced in an emergency. Also in an emergency situation the hospital liaison committee may be asked to attend the hospital and their aim is to ensure that the witness maintains their decision not to have blood.
If your wife knowingly accepts a transfusion she disassociates herself from the witnesses so there is a great deal of pressure to comply.
This is a dangerous policy that witnesses follow because they think they are respeting Gods prohibition on blood. It was actually introduced to bring witnesses into the news and to increase publicity for the organisation. People die for the sake of advertising.