The District lets your policy bar stacking, but does not require it
The District's rule is in D.C. Code 31-2406. Subsection (f)(7) lets a policy preclude stacking of uninsured motorist coverages, subsection (c-1) does the same for underinsured coverage and makes it optional, and the definition of an underinsured vehicle is in 31-2402.
Read how stacked and unstacked coverage differ in the District, check what the statute says and leaves open, or see how mandatory UM and optional UIM work.