Starting with the SBD and adding a pair of mics to improve that it is more or less the opposite approach to the more standard practice around here of starting with a well optimized AUD and adding SBD for clarity. So perhaps it's not that surprising that very different mic locations and configurations can work or actually be called for.
I attended a friend's wedding in Santa Cruz a couple months ago and ended up being asked to run sound for his band performing in a big redwood barn at a retreat in the mountains. Soundboard was located off to one side of the band due to the length of cables. I had brought a small rig to record their performance, but the recording immediately became of secondary importance. Once we got them all set up and soundchecked, I quickly made an aux bus SBD mix and recorded that to a DR2d along with two dpa 4060 omnis boundary mounted to adjacent sides of a huge redwood beam against side the wall behind me- one side facing directly towards band, the other along the wall into the audience side of the room. Chose a mic position that was close, easily accessable, within the length of patch cable and out of the way. Checked levels and just let it run, hands full with lots of open mics and acoustic instruments during the performance. I didn’t worry about it too much and
When I got home the next week and transferred the files, I realized I had neglected to reset the recorder to write wave files again after letting my nephew record some things as 128bit mp3 files a couple days before the wedding. Between the compromised mic placement and the low rate mp3 format I didn't expect much, but the combination of the SBD and AUD actually worked out quite nicely. Neither alone is great, but the combination of them work very well, exactly as I hoped it would. Still need to mix it down for them, maybe this week.
If you have a SBD feed, the requirements of what you need from a pair of AUD mics is relaxed. You can get away with a far less than ideal placement since you really just need the room ambience, audience reaction, and any musical information that isn't in the SBD feed. In my situation I had all instrumentation and vocals in the feed through the board, so I just needed good room ambience, depth and audience reaction from the AUD mics behind me on the column to glue it all together and put everything in place.
Similarly, when making a multitrack recording off the board, its common practice to use two very wide-spaced cardioids, each at opposite, far edges of the stage pointing out into the room as a way of picking up audience reaction and room ambience while limiting pickup of the sound on stage.
The other (opposite) way to go about it is to treat it as a more typical AUD + SBD and try to optimize the AUD as if there would be no SBD available. Which means getting the mics arranged on stage or at the stage lip in a typical stereo configuration like the others have described here.
While somewhat opposite approaches, both can work. It somewhat depends on the music, how good the SBD feed sounds, and what you can get away with doing in a practical sense.