Thumbitextrous 

Able to text with both thumbs equally well.

If Elvis had lived to see the use of cell phones he would have undoubtedly been thumbitextrous.

Thumbitextrous by El Guapo Chalupa May 23, 2012

Thumbidextrous 

The ability to use both thumbs equally well for texting and/or mobile gaming

Though not ambidextrous at all, since acquiring a cellular phone, Dan has come to be thumbidextrous.

Thumbidextrous by DragonnHAIL! n September 12, 2016