The booking page URL can contain a number of parameters to set how the selectors should initially be shown.
The full set of parameters is described on the widgets >> web designers page.
To change the number of nights selector to 2 you add the numnight parameter to the URL which opens the booking page.
Depending how you are opening the page this is either a hidden variable on the form or appended to the end of the URL.
For example the following link will open our demo property with 2 nights to be booked:
https://www.beds24.com/booking.php?prop ... numnight=2
This example will show 7 nights as the initial value
https://www.beds24.com/booking.php?prop ... numnight=7