I'm just a grad student, so take this with a huge lump of salt, but a lot of the advice I've seen here for professors writing these letters is to include specific examples of things the student has done, so like instead of saying, "Suzy is a brilliant collaborator with peers," say something like, "Suzy led an interdisciplinary undergraduate project in XYZ, which led to [result]."
If you have specific accomplishments that the professors would know about (hopefully not the exact same ones for both professors), then maybe that's a place to start. Maybe one was your thesis advisor and can be expected to write about your thesis while another one saw you do some other thing.
That could help differentiate the letters and also allows you to write about factual matters rather than about how you're the best thing since sliced bread or whatever.
All this.
I've had several folks who told me to write my own letter. One way to look at it is that they're lazy--but an entirely different and more generous way to look at it is that they expect you to be your own best advocate.
You know what the opportunity (job/postdoc/grant/etc.) is, and how you qualify for it.
You are in the best position to articulate how you "fit."
You are intimately familiar with your accomplishments and how they have prepared you for this opportunity. Etc.
So, go ahead and write the best letter that you can for yourself. As ptarmigan says, be specific. A careful selection of which facts about yourself to emphasize (specific accomplishments and goals, successful experiences, etc.) will speak many more volumes about your qualifications and fit than simply stringing together superlatives that make you uncomfortable to write. The two letters should indeed be different, so think about how each professor knows you (context, etc.) and what aspects of your qualifications/fit/etc. each is best positioned to address. Then have at it.
Good luck!
