Having a vehicle of any value on SSI is allowed. When you own a second vehicle that vehicle is counted as income and becomes a resource which could effect your ability to continue receive SSI.
If a spouse works and a second vehicle is needed for making it to doctor appointments or for emergencies, that vehicle has to be really cheap. Really cheap vehicles have a problem because they take people whom already qualify for SSI and make them drive a vehicle that is prone to problems. Cheaper vehicles are older with higher mileage and this creates a financial burden for those whom are the most financially vulnerable.
Something needs done for those who are already financially burdened enough.