If you need the changer part of that connector then I wouldn't try cutting it in two, much easier to tap the line-outs and remote on at the amp.
I take it you have the non Bose setup, so the amp is bolted to the rear speaker. So cut of one end of a pair of RCA's, strip them back, connect the screens together and you'll then have 3 leads (bare screen, red & white signal leads). You'll also need a blue remote on lead. Unplug the lead going into the left speaker and use some piggyback connectors to make the connections (or better still cut the wires you need and use a terminal block to reconnect them and pair them up with the RCA lead). Piggybacks are not the most reliable method.
Connect your blue remote on to the white wire.
Connect the RCA screen to the brown wire with red stripe (check this is 12v and switched on/off with the HU)
Connect the RCA white wire to the blue wire with green stripe.
Connect the RCA red wire to the blue wire with red stripe.
Hopefully these colour codings are the same in your A4, Audi are renowned for changing them on a weekly basis. As long as you get the remote on wire correct and you steer clear of the permanent live on the amp you can't really do much damage.