A Ground Controller noted a conflict between a helicopter and Cessna crossing the airport apparently unnoticed by the two Local controllers. The reporter recommended changes to the SOP procedures to prevent future events.

Narrative

I was working Ground Control. Traffic volume for the Local Control 1 was busy and traffic volume for Local Control 2 was moderate. The CIC made a comment to the 2 local controllers that alerted me to look at the situation. The CIC was asking the local controllers what was being done about a Cessna and a helicopter that were crossing the field midfield side by side at the same approximate altitude. When the CIC questioned; the two controllers issued traffic to their aircraft. The CIC asked what the altitudes of the 2 aircraft were and I observed via the STARS that the Cessna indicated 2;000 FT MSL and the helicopter indicated 1;800 FT MSL. I estimate that the 2 had approximately 100 FT lateral separation. The two aircraft got into this situation because the Local Control 1 Controller allowed his arriving Cessna to descend unrestricted from the north to cross over the field for right traffic to his Runway XXR. The Local Control 2 Controller had the helicopter on a transition from north to south; which put him at 1;900 FT by LOA. Recommendation; the SOP stipulates all aircraft will cross overhead at 3;000 FT; unless coordinated. I recommend that the option; through coordination; to descend aircraft crossing the field be removed from the SOP. There is no reason to take the risk of an aircraft descending through the other Local Control's airspace becoming a factor with that controller's pattern aircraft at 2;400 FT or helicopter traffic at 1;900 FT.