Getting a proper balance between both roles having something to do would be rather difficult but it could be done. There would only be one rescue tech so they would either lack manpower or have to rely on atmos for support. They also wouldn't have atmos access or access to to really any bits of atmos equipment.
I can see the role functioning alongside atmos techs. Rescue techs would play a frontline role (gaining access, doing the brunt of firefighting, helping treat/rescue the wounded) while atmos techs would either stay in atmos manning the atmos alarm computer or help support the recue tech (managing air alarms onsite, using cans/airscrubbers and pumps)