These are not well-known information, but Pixel 2 and 3 can technically work as dual-SIM devices thanks to their eSIM support. eSIM is used by only a few carriers around the world, including Sprint in the United States, but can function as a second SIM card next to the SIM card. However, Google seems to improve dual-SIM in upcoming Pixel 4.
There are several different dual-SIM versions: Dual SIM, Single Standby (DSSS); Dual SIM, Dual Standby (DSDS); and Dual SIM, Dual Active (DSDA). Pixel 2 and 3 support DSSS, which means that two SIM cards can be provided in two networks within the phone, only the active SIM can receive calls and texts. DSDS is what most phones use with a dual SIM card and the latest iPhone; In this case, the other SIM card may receive calls and texts if the main SIM card is not currently used for the same purpose. Lastly, DSDA means that both SIM cards can be used for calls, texts and data at the same time. Because it requires two radios, it's not very common.
According to a recent commitment to Gerit AOSP, Pixel 4 will have dual-SIM functionality. The same decision mentions that Pixel 3 also supports the use of two SIM cards, but is limited to food. XDA disguises that Pixel 4 will support Dual SIM, Dual Standby (DSDS), which is definitely a logical ending. It's possible that Pixel 2 and 3 will be updated with this enhanced performance, even if you do not secure any bets on it.