The main difference between benzoyl peroxide (BPO) and salicylic acid is that benzoyl peroxide has antibacterial properties, while salicylic acid doesn't. Both BPO and salicylic acid bars can be used in rotation (or at the same time). Salicylic acid works best for blackheads and whiteheads (e.g. clogged pores). Most folks use BPO to gain control over severe acne outbreaks and then switch to salicylic acid for daily maintenance once the acne is more manageable. If using both actives together in rotation, use with caution. The combination can be super drying. Try not to use too many products at the same time. You need to give the redness and irritation time to heal (10-14 days).
It is not permissible to manufacture a bar with both BPO and salicylic acid. If you would like to experiment with using both active ingredients, you'll need to buy both bar types.