I have done this many times over the past year, but as follows:

- upon entry to USA, I mentioned that I was planning to go to MEX after some business in the US, received a stamp in my passport.
- upon exit from USA for MEX, gave up my I-94 form to the airline.
- when reentering USA, filled up a new I-94 form and received a new stamp in my passport.
- upon departing USA, gave up the new I-94 form to the airline

No issues at all. (well, none except for some "attitude" from one immigration official)