The answer is "it depends". It certainly won't improve your chances, so at best it's no effect, at worst it hurts you.
It depends upon how alike the two fields are.
It depends upon which one is more competitive.
It depends on the program.
Let's use the example of IM and Neuro, since you mentioned it. The two fields are rather similar, so that's not a problem. You could imagine that if you applied to Ortho and Psych, both would think you crazy for applying to the other and that wouldn't go over well.
If I find out you applied to both, I'm certain to ask which one of them is your first choice. It's a perfectly valid question. Most people will assume that IM is your first choice, since it's seen as more competitive than neuro. If you tell either field that you're their backup, they're likely to consider not ranking you -- last thing anyone wants is someone who is unhappy. You might say "but I'd be happy in either field", and we're happy to tell you about all the stories of people who told us that, and then 3 months into PGY-1 want to switch out.
So, if you tell IM programs that IM is #1 and Neuro is #2, that's relatively fine. Programs might still say that it shows a lack of committment to IM. The problem is that on the rank list, lots of candidates look very similar so something like this can move you down slots, with a program figuring that they would rather have someone who is 100% committed to IM than someone who is less than 100%. Then again, some programs may not care at all.
The other way is complicated. If you tell Neuro programs that they are #1 and IM is your backup, they will look at you funny (since IM is more competitive than neuro) and likely distrust you, and figure that neuro is your backup. Even so, because neuro isn't as competitive, they might rank you anyway. But it could be a problem.
Bottom line -- you can't tell, unlikely to be a "death knell" with Neuro/IM combination.