Yes. The simple threshold for nested rides is: Did you by default have to do the lesser ride in order to do the greater ride? You can't nest a SS1K inside a BBG1500 because by default, you had to have completed 1000 miles in 24 hours in order to complete 1500 miles in 24 hours.
Conversely, with the SS1K and the BB1500 rides, you could ride less than 1000 miles in 24 hours while still completing the BB1500 in 36 hours, so if you documented the 1000 miles in 24 hours, then finished up the additional 500 miles within 36 hours of your original start, you are allowed to claim both certs.
Be aware though, for the purposes of the Mile Eater certs, nested rides are only one "ride". Usually the longer one counts toward your Mile Eater. Since both the SS1K and BB1500 are considered basic rides, not "gold" rides, it doesn't really matter which is used for the Mile Eater count.