In reply to djwilse:
Address them by honorific and surname - check the title (Mr or Dr), then expand on the why in the letter:
Dear Mr X and Dr Y,
I am contacting you in your respective capacities as the current Head Master and future Head Master of XYZ. &c. &c. Yada yada
I would highly recommend having a 3rd party check your spelling and grammar, if it's a fifth as bad as mine... Also to make sure what is clear in your head is clear on paper.
Post edited at 22:20