How can I get rid of 2 dark spots on my buttock? (Photo)

This is likely a combination of a few things, there arent many good treatments for this given that you sit alot and wont change that going forward.  Chemical peels along with lightening creams and shower scrubs will be helpful.  I suggest seeing a cosmetic dermatologist who has expertise in treating pigmentation.  Best, Dr. Emer.

I would use Luminase and Exfolase creams followed by fractional laser to help reduce the pigmentation on the buttocks. Best, Dr. Karamanoukian#Realself100 Surgeon

Chronic irritation, as in sitting on hard chairs can cause hyperpigmentation of the buttocks. Also, dark skin is prone to hyperpigmentation.
Chronic moisture can cause skin irritation and result in hyperpigmentation of the inner thighs , posterior thighs and knees, underarms, buttocks and back. 
Before you spend $$ on peels or IPL, try these topicals.
This is called post-inflammatory hyperpigmentation (PIH). 
I recommend Melarase AM and Melarase PM creams from Kare Skin as well as a broad spectrum sunblock like Spectrase. Bothe are from Kare Skin. Scler-x post inflammatory hyperigmentation relief complex is a great supplement to take as well. See link below. 
Spectrase is a wide spectrum sunblock SPF 50+that is also recommended to prevent ongoing inflammation. 

One of the best ways to treat these spots are lightening creams with hydroquinones combined with chemical peels.  Please consult a dermatologist who has expertise with pigmentation and peels for the best cosmetic results.