Yeah totally, nothing stopping you. Most affiliate programs don't have exclusivity clauses so you can promote as many competing brands as you want.
Actually makes a lot of sense strategically - comparison content, "best protein powder for X" roundups, honest reviews testing both. That stuff converts really well because it feels genuine rather than just pushing one thing.
From my side managing nutra publishers - when someone tells me they're running similar offers across multiple networks that's not a red flag at all. It actually tells me they know what they're doing. The best affiliates I work with are always testing multiple angles and offers simultaneously, that's just smart business.
Ranking/comparison pages in particular are something I always push my publishers toward - high intent traffic, great conversion rates, and you can monetize multiple brands on the same page. Win win.