Survivor benefits when both parents passed away

This is possible, but relatively rare. In most cases, a single benefit based on the higher wage earner's history is granted.

Excerpt from http://www.ssa.gov/oig/ADOBEPDF/A-01-05-25015.pdf :

Individuals may be entitled to auxiliary/survivor benefits based on several workers' earnings simultaneously (for example, based on the earnings of both parents), but may generally only be paid on the higher of the two. When an auxiliary/survivor beneficiary becomes entitled to another, higher benefit, the Social Security Administration’s (SSA) policy is to stop issuing the lower benefit payments, thus preventing an overpayment from occurring.