An smartphone keyboard interface for swiping with thumbs
There are different keyboard input methods for smartphones:
Compact (T9) typing
Double input swiping has been explored very little. The only keyboard that I have found to implement a good working solution is keyboard69.
Why is it so uncommon?
Implementation is hard. Multiple input means there are async processes filling an input buffer that has to be queried for completion candidates on each new event.
It requires some learning from the user, this make it unattractive, since the higher the learning curve the lesser the user retention.