Yes, the Galaxy S8 and Galaxy S8+ will have two SIM slots in some markets, following in the footsteps of the Galaxy S7, S7 edge and Note 7. Samsung is yet to confirm which markets will receive the dual SIM variants, but if you need to check, simply head over to Samsung's website for your particular country and see the Galaxy S8/S8+ listing. If your market is getting the dual SIM variant, the phones will be listed as the Galaxy S8 (Dual Sim) and Galaxy S8+ (Dual Sim).
Dual SIM smartphones are popular in Asian markets, so the dual SIM models should come to countries like India, Dubai, and China. Samsung hasn't mentioned if the S8 and S8+ will have a hybrid slot or separate slots for two SIM cards and a microSD card, but we're certainly hoping it's the latter given the company has a lot of real estate to work with on its new flagships. The batteries aren't very big on these handsets, either, so fingers crossed that having to choose between two carriers or a microSD will be a thing of the past.