Two-lane streets are a special kind of road that are used in later cities and have some special conditions for forming crossroads. More specifically, a crossroad can only be formed every other street segment, not on every segment of the two-lane street. Keep this in mind when planning your city's infrastructure!
Please also remember that two-lane streets always require a direct connection to the city hall, so it is not possible to connect two two-lane streets using a one-lane street. Additionally, some buildings may require a two-lane street connection to function. You can see if this is the case in the building information of the building menu.