Delaware keeps one insurer's limits from adding up
Delaware's rule is in 18 Del. C. 3902. Subsection (c) says that insuring more than one person or vehicle does not raise the insurer's limit and caps same-household vehicles with one insurer at the highest single limit. Subsection (b) makes underinsured coverage an optional addition and defines an underinsured vehicle by comparing the at-fault limits with your damages.
Read how stacked and unstacked coverage differ in Delaware, check what subsection (c) says and what it leaves open, or see how UM and optional UIM work.