Bosses can be really good at seeing some things and missing others.
SEOs can be really good at seeing some things and missing others.
I assume that you were hired for your expertise and that your bosses don't know much about SEO. So, if you do not agree with them you can show your value to the company by letting them know why you think that these sites should remain separate.
If you decide to do this do it in a very nice and business like way. Don't rant, just present data - and sometimes that data will be what you "think" will happen which could be wrong.
If my company bought a website we would not merge it immediately even if we wanted to. Instead we would run it for a while and get to know where its traffic comes from, how much money it makes, where its power flows from, etc.
We might decide to merge it or we might run it separately.
One advantage that I see for merging the new site into the original is to bring that traffic and power to the corporate site. There the visitor gets a much wider selection of products and the company has a better ability to market their brand to these visitors. And, if the original site is not dominant in your niche this extra power might push it there. But, if the original site is already dominant then the acquired site displaces a competitor.
If the acquired site outranks the original site maybe adding other products to that site would supercharge its revenue.
To determine if the hardcore traffic of the acquired site is receptive to the brand of the original site you might run some ads on that site to see if they are ignored or if they are clicked through. If you get lots of clickthroughs and those visitors engage the site and purchase then merging the site might work well. But if they ignore the ads or bounce that says something too.
I would be running some tests and getting some data before I do anything. And, anybody who says do this, or do that, is guessing. Add testing to your proposal to the bosses. Tell them that there are ways of measuring the value of this acquisition and if the traffic of the site is valuable to your brand.