Side-by-side windows are simply not used by normal users, which the vast majority of websites are designed for. The real question here is why don't you have a second monitor?
The mindset around here is "everyone should design things for my extremely specific use case that 99% of users don't share". Find me stats about the % of users with desktop browser windows narrower than 900px and then we can talk.
The mindset around here is "everyone should stop overdesigning their sites so that they only work for people in the peak of the bell curve and needlessly exclude all the less-common use cases, of which mine is an example".
The web works better when site designers try to exert less control over it. Who are you to tell me how wide my browser window should be?
Design responsively and assume that every size of window is equally likely at some point in time. Just because a use-case isn't common, does that mean your product or website shouldn't be usable on it?
Most of those visitors are likely coming from HN, a community of people who are more likely than most to have multiple monitors. For the record, I multi-monitor at all my stationary worksites. But I don't carry external monitors on planes, airports, coffee shops, etc.