It CAN be done. Although co.com can get quirky about it.

I once had a same day turnaround MR to PHX and had no problems.

This past weekend I did a EWR-CGN turnaround and Co.com wouldn't let me check in for the return flight at the 24 hour window but did when the local time in EWR was exactly the same as the time of departure from CGN (i.e. at 11am EST on Saturday for a flight departing from CGN at 11AM CGN time on Sunday [18 hours prior]).

The other trick I found with getting the online check-in to work was to NOT sign in to my account. Rather, from Co.com's home page entering the confirmation number. Any other way and I kept getting error messages that said co.com could not find a valid e-coupon...or something like that. Both online support and reservations could not tell what was wrong and confirmed that my reservations were in sync. Defintely not a SSSS thing because I didn't have any troubles at the airports. Even after reprinting my boarding passes at both airports.

I have another immediate turnaround EWR-CGN this weekend...will report back if I encounter the same problems or if I get shut out from online check-in for the return completely.