4 Audio Channels

I'd like to set up a project in my FCPX that allows for 4 channels of audio.

CH 1 - Left
CH 2 - Right
CH 3 - Mono Sum of CH 1 & 2
CH 4 - Mono Sum of CH 1 & 2

When I set up my project, the only options I see are for stereo and surround.

I haven't seen that configuration since using tape for broadcast delivery. Wow, who does that these days?

Viacom Inc. - Media Delivery Technical Specifications for US Network Operations.

"If neither SAP nor DVS are delivered, a mono mix of channels 1 and 2 must be delivered on Channel 3 and 4. Note: It is NOT acceptable to copy the Channel 1&2 stereo mix to Channels 3&4 - rather, a mono sum of 1&2 must be created and delivered on 3 and 4. This is to ensure that viewers who listen to SAP audio receive a complete audio service on their mono SAP signal, not simply “half” of a stereo signal."