\bool_new:N \l__L_bool
\bool_new:N \l__R_bool
\bool_set_true:N \l__L_bool
\bool_set_true:N \l__R_bool

\keys_define:nn { mymodule }{
    L .bool_set:N = \l__L_bool,
    L .default:n  = {true},
    R .bool_set:N = \l__R_bool,
    R .default:n  = {true},
}