The two apart facts are also called Doubles +2, Monkey in the Middle, or Robin Hood Facts.
A fact like 7 + 9 can be thought of as ‘Double 7, + 2 more’. This strategy relies on knowing the doubles and the near number +2.
The monkey in the middle naming comes from the idea that any two addends two apart can be found by doubling the middle number. For example, 7 + 9 is double 8.
Robin Hood is much the same, and refers to robbing from the ‘rich’ or higher number, and giving to the ‘poor’, or lower number.
Robin Hood Flash Cards
These cards show the addition fact on two ten frames, with an arrow indicating how the fact can be made into a double.