Yes, your inXile backer account will work on multiple games. If you sign up for a backer account, your email and password will work for the following:
You do not need to create a new account for each game you backed separately, however, you can visit the Email Addresses page on any of the backer web sites to link multiple secondary emails onto the same account.
Note: Your backer account will not work for Wasteland 3, as Wasteland 3 is using CrowdOx, which is a separate backer rewards web site. CrowdOx does not currently have account logins; instead, you can view your pledge details through a unique URL sent to your inbox, and you will receive updates, news and rewards via email.