The RS-603 – 2 channel pack uses the TWC-701 to allow both channels to arrive on one –two pair shielded microphone cable.  
The RS-603 can send either DC call signal "Standard Clear-Com mode" or AC call signal "standard RTS mode"
NOTE: RTS-compatible beltpacks—the RS-603, RS-623, RS-603R, and RS-623R models—offer DC call signaling only on the B channel. If you use the Channel Reverse feature , DC call signaling switches to the A channel and deactivates on the B channel. AC call signaling will not work with the RS-601 or any RS-500 series beltpack.
When calling from RS-603 to RS-603, the call signal lives in the 20KHz world..  thus – calls between the packs works...on both channels.
However, the MS-702 does not recognize 20Khz thus, (it only uses the DC call signal for both channels)... no call between both channels --- only one.

The pack you want is the RS-602 --- it will call on both channels to/from packs and the MS-702.
Yes, it is a 6 pin – not a 3 pin thus you will need 2 pair shielded mic cable or 2 single pair with the YC-36.   You will not need or use the TWC-701