Combining multiple merchants' products on a page or site
As an affiliate, I've frequently mixed products from several merchants with affiliate programs, on the same page.
And the price-comparison sites all combine offers from different companies on a single page.
Certainly, it is important to make clear which products are being offered by which merchants -- you would never want someone to think they were buying from merchant "A" and then find themselves at the product page for merchant "B" -- and avoid mentioning special terms like "free shipping" or "15% off orders of $20 or more" if it's not clear which merchant(s) the offer applies to.
As noted, you need to check the program terms for each merchant.