How do I bond units?

First, you need to have TWO DBB-eligible units (Which units are DBB-eligible?) which are DBB-unlocked (How do I create a DBB-unlocked unit?)

View EITHER of the DBB-unlocked units and tap on its newly unlocked DBB SLOT. Select its partner.

Once bonded, their DBB slots will be filled with a "BB" icon of their partner.