Having a business specializing in two different services

I currently use a marketing company for company A and have been using them for a few years now, they’re great and have helped me grow exponentially. We recently decided to get into business B and had them draw up a proposal and marketing plan for the company. They will both operate under the same name and I was under the assumption we would just add another landing page on our current site. They recommend to build out a separate site for business B and listed SEO as their main reason for this. Any advice for me to make a smart decision here ?