Is 3500 USD (12740 QAR) sufficient for 2 people in Qatar?

depends of your standard of living , the amount is relatively ok, for 2 people , not really sufficient for family with kids ,you excpected to spend around 3000 to 4000 rials for housing , 2000 for different costs , so with good cost saving, u can save 1500 usd per month

that amount is the most average salary paid in doha , like 60 % or more of the workers are paid around that amount , so make your calculation and decide

this is more than enough for two adults to live in qatar peacefully.all the very best.