how to start one letter to two people?

Question

Hi all,

I would like to write a letter to two people. Shall I start like:
" Dear Professor X, Dear Professor Y"
or:
"Dear Professor X,
Dear Professor Y, "
or any other forms? What's the correct format?

Thanks:)

Answer 1

Dear Professor Dillon and Professor Brown,
I have so much enjoyed your lectures etc etc. ..

Answer 2

Dear Professor X,
Dear Professor Y,

Is the correct format when writing a "business letter" addressed to two people.

Answer 3

Dear Professor X & Professor Y: