In general Investors have a rule that they don't "back-fill" past liabilities, and that's what you are asking them to do here. However there are exceptions to every rule.
So I think trying to get your full salary fully back-filled would indeed be a huge turn off for any investor because of the message it would send. As others have stated, you are really trying to have your cake and eat it. On the other hand most investors don't want you so broke that your personal financial problems are impeding your ability to execute. So it would be reasonable to say "look after 2 years of no salary I am really struggling financially and a one off payment of $X would really help me focus on the business". Thats a much more acceptable message.
I don't know how much $X should be but its likely to be a lot less than 24 months salary!