"What is the difference between stacked and unstacked uninsured and underinsured motorist coverages?
Unstacked coverages protect up to the stated limits on the policy. Stacked coverage allows the uninsured and underinsured limits to be multiplied by the number of vehicles on the policy. For example, if you choose stacked coverages with two insured vehicles your $100,000/$300,000 coverage is increased to $200,000/$600,000"
from
http://www.lawtonins.com/faq.aspx#7