I try never to let on that I own both because inevitably a buyer will want one thrown in. In fact I make every effort to assure I disguise one of them so that it looks like two different owners.
That maximizes your resale value because the purchaser has to deal with two different entities (hypothetically).
In fact I once sold a singular and the plural domain sold under my wife's maiden name to the same purchaser about 9 months later. It was a funny situation because we even used the sale price of the first domain to justify the price of the second.
So my advice is do not let on you own both, do so only if you are sure you can get near equal selling price.
I have two one word .com's, the singular and plural of the same word. One I got in the early 2000's, the second I got a few years ago from one of those outfits that targets you because you own a similar domain. I created a simple sales page for both of them (both domains listed on the page) with a contact form and price in the xxx,xxx range for each. Kind of my domain lottery ticket, I'm encouraged by the prices being paid for one word .com's in the past year. And for that price range, I don't mind giving a potential buyer a deal if they take both. I figure if a buyer wants both and knows you own both, it may make them more encouraged to get a deal done. Especially if they are considering other names where they know it would be harder to get both.
I see MapleDots point too, hiding that you own both names to try to maximize the sales, I just prefer what I do.
My one word .com's aren't super premiums or household words, but they aren't super obscure either. When you see a domain like Recursion.com selling for $900,000 US this year, they why not increase your asks.