The same two letters, in the same, but not consecutive, order, when added in their appropriate places to the words below, will create new words - two of which are  proper nouns.

SUN
SINS
PINT
THEN

HINT:
The order doesn't matter, because the two letters are the same.

Answers:

The two letters are AA:

SAUNA
ASIANS 
PINATA
ATHENA

Credit: NY Times Crossword
July 22, 2012
Brendan Emmett Quigley 
A.A. MEETINGS