Two company aircraft with very similar call signs departing parallel runways simultaneously. First aircraft checks in on the wrong departure frequency and is issued a heading intended for second aircraft causing TCAS RA.

Narrative

I was the pilot flying and the Captain was the pilot monitoring when we were cleared for takeoff on Runway XXR at ZZZ. We took off with a 180 degree assigned heading and climbed to 5000 FT. Our flight number was Flight ABC. Almost simultaneously; flight BAC was departing the parallel Runway XXL. As briefed; I was hand flying the aircraft until reaching 10000 FT. After takeoff; Tower told us to contact Departure. The Captain changed to Departure frequency and Departure acknowledged our call and stated; 'Flight BAC; turn left to 090 degrees and climb and maintain 16000 FT.' Neither the Captain; nor I; nor the Departure Controller were aware of the discrepancy. Departure was expecting the other aircraft; flight BAC to check in. I began a left turn to 090 degrees and started to climb to 16000 FT. As we began to roll out on the new heading and passing approximately 3000 FT; we received a TCAS Alert; 'Traffic; Traffic.' I saw the amber target on the display and began searching for the other traffic. We were between layers at that time. Within a few seconds we received the TCAS Resolution Advisory; 'Descend; descend.' I stopped the climb at approximately 3200 FT by lowering the pitch and reducing the power; and then began a descent. We saw the other aircraft and it appeared to be about a mile away and higher. At that time; Departure became aware of the situation and told us to level off at 3000 FT and turn right to 180 degrees. We were subsequently cleared to 16000 FT and cleared direct to our first fix. Two factors contributed to the situation. First; even thought our flight plan has SE outbound course; I had incorrectly tuned the frequency for the East departure. The departure shows a radial that indicates that we were to use the frequency for West departure. These frequencies are not stamped on the sectors of the departure page like many other large airports; but instead are simply listed at the top right corner of the page. Another contributing factor was similarity of call signs. As I mentioned above; none of us (the Captain; myself; nor the Controller) realized that there was a discrepancy until we received the TCAS Alert. Finally; since we were expecting a left turn on course at some point; it did not seem unusual that we were given instructions to turn left to 090 degrees; especially since there were very few departures off the left side. Clearly; greater diligence is listening carefully for call signs will prevent a recurrence. Also; call signs should be de-conflicted so that there is no possibility that two aircraft with similar call signs are in the same airspace during critical portions of a flight. Additionally; the departure page should be revised so that the departure frequencies are more easily understood. An aircraft departing ZZZ on a SE course should not be assigned or expected to use a 'West' departure frequency. Supplemental info from ACN 774975: I was the Captain departing ZZZ. First Officer was flying. Tower cleared us for take off from Runway XXL. 1 mile south of the VOR fly heading 160. After passing the VOR was instructed to contact departure. Autopilot was engaged per air carrier procedures and heading 160 was selected. 1 mile south of the VOR I switched to departure at that time I heard; 'Flight BAC fly heading 090.' Another aircraft had replied; I had not contacted departure at that time I had just made the frequency change the First Officer started turning to heading 090 I told him to wait. I felt there were some call sign confusion I had not made contact with the Departure Controller; about the time we were returning back to heading 160 we got the TCAS RA. First officer disconnected the Auto Pilot and started hand flying with an aggressive climb per TCAS Climb Command after a few seconds we were clear of conflict made positive contact with ZZZ Departure control and advised him of the RA and we were given heading 090 and climb to 16;000 FT; at that time I advised departure control we had to return due to