Let me answer this from a somewhat different perspective. I'm an industrial researcher and have written a fair number of letters for former interns applying to faculty positions or sometimes other research labs. I always write them myself and focus on on the experience I had with the student. And I definitely wouldn't want them putting words in my mouth.
But I've written a number of support letters for visa requests too. These are typically extremely flowery prose, talking about how absolutely wonderful the applicant is. And I hate to write such a letter because I know I won't do it in the same style that the immigration lawyers write it, and I worry my own letter won't be helpful to their cause. So in the rare case that they don't provide a template, I ask for one. Then I edit it to make sure I agree with what's said.
The closest example to the case at hand is when someone who has worked in my office asks both me and a colleague to provide a support letter for the same application. We're both asking for a draft, and the poor guy has to do exactly what OP here has to. To the extent that we describe our joint involvement, we're both going to describe the same project, and there is only so much they can be distinguished.
This case is a bit different, though, because such letters go back through the applicant. It means if we don't diverge enough after the recommenders edit the provided example, they (or their immigration attorney) can at least make a determination that more has to be done.
Getting back (at last) to the comparison with a faculty letter, it seems to me there are two questions:
- How do you make the "voice" of these letters seem different enough that they're not apparently written by the same person? This question applies to the immigration case too, and I think the same answer applies: if you can't figure out how to make it different enough on your own, find a trusted advisor who can take the framework (similar to the answer by @Brian-g-Peterson) and rework it on your behalf.
- How do you make sure that in the end they aren't too similar? One aspect is to make them as different as possible in the first place. Cover some points with one recommender and others with the other. Also, if there is any opportunity to review the letters, or have someone else get permission to review them on your behalf, they can be sanity checked.
Bottom line: I assume whoever takes the letter is likely to use it only as a starting point, but much of it (and possibly all of it) may survive. If they overlap, even with one (not very commonplace) sentence, it will be a huge red flag. Saying "I give XXX my strongest recommendation" is common. But not much else, if in both letters, can be explained away. Tread carefully. Or find other recommenders who'll do it themselves :)