A80 Controller described a near loss of separation event when ATL Tower Local positions apparently mis coordinated transitioning to headings vs. RNAV procedures.

Narrative

ATL Tower departed a DC10 off 26R and a B757 departed 27L. It was at the end of the day; and I believe the Controller on Local Control 2 rolled the DC10 on an assigned heading; while the Controller on Local Control 3 rolled the B757 on the SID. I was coordinating with the Controller next to me when I noticed the two aircraft wingtip-to-wingtip and I turned my aircraft south. The Local Control 3 had called down and told me that was their last RNAV Off The Ground; and they were going to be issuing headings after that aircraft when the Tower goes to heading for noise abatement at night; 15 degrees is ensured; when aircraft are on the RNAV departure; they're separated by divergence; in some cases reduced divergence per procedures/waivers at ATL. They could've been on the RNAV departure on the North complex; but I don't believe they were. The resultant separation was less than standard. I turned my aircraft as soon as it occurred to me there may have been a miscommunication in the Tower. Procedures say we go to heading a lot earlier than ATL normally does due to convenience and volume. If what it appeared happen did actually occur; this is a coordination problem in the Tower and needs to be worked out.