Hi Alex,
Yes, there is a battery that powers the device while in Tablet mode and another in the base when it is used in Laptop mode. In Laptop mode these batteries work together to provide the longest battery life and, when charging, will automatically switch from one to the other. You can check on the battery charge progress by clicking the battery icon in the taskbar. For more info on charging your devices, please view Surface battery and power article.
For more info on Surface Book 2, please view Surface Book 2 Tech specs and Surface Book 2 fact sheet.
Should you need further assistance, feel free to get back to us.
Regards.