In most cases I sold the .com and the buyer registered the available .net. I generally only register a .net for a name where I intend to develop both the .com and .net sites. In fact I usually prefer to leave .nets and hyphenated versions of my .coms available. This way someone can register and develop them. When they do I get a lot of their traffic and email because most people try the .com name before .net. This drives the value of the .com up even more.
There are still a few people out there who feel the need to have the .net for every .com they own but I think most people realise this whole โmust haveโ thing was a marketing scam designed to sell registrations. Still, if you can find someone who still believes that someone will actually type in a .net rather than the .com they are probably crazy enough to pay 1/5 of what the .com is worth