The dual SIM support is interesting for a couple of reasons.
We're nearly certainly going to get Android Q arriving with the Pixel 4, ready to take advantage of that dual-SIM functionality. But later in the year, the company will likely launch the new Pixel 4 line sometime around October. Like Apple's newest iPhone, those Pixel phones have a tray for one SIM card while the second is provided by an eSIM.
While the Pixel 2 and the Pixel 3 can technically support two SIMs through eSIM technology, it's not the more advanced version of the tech available in many other handsets, including the latest iPhones.
Like those older dual SIM phones, the current Pixel use Dual SIM Single Standby or DSSS. Most Android smartphones come with Dual-SIM, Dual Active (DSDA) support, meaning users would be manually able to choose which SIM to be used for calls and data. You'll have to switch between the two networks to receive calls and messages from the respective network. In other words, you can't have simultaneous calls on both SIMs (though why you'd want this I dunno). DSDS is what's used by most dual-SIM Android phones and the newest iPhones; here, the second SIM can receive calls and texts if the main SIM isn't being used at that moment for the same goal. Ideally, phones would have Dual SIM Dual Active (DSDA) capabilities but those require two wireless radios instead of one.
The Google Pixel 3 series of smartphone may have been out on the market for less than six months, but that isn't stopping the search engine from marching forward with the development of its successor. You can not use Dual SIM, Dual Standby to be on two calls at the same time, or have two text messages going on simultaneously. The Googler also points out that Dual SIM, Dual Standby is being tested internally on the Pixel 3 in preparation for use with the Pixel 4 models.