Our best acoustic sounds were usually achieved in studio using two (or more) mics, usually a large diaphragm tube combined with a small condenser mic. Summing mixes to mono was a standard check and often revealed phasing issues. Flipping the standard console 180* switches sometimes provided a livable result, but was oftentimes unsatisfactory, as mentioned above.
Then, Little Labs introduced the IBP box. IBP stands for In Between Alignment. Brilliant!! One now had continuously variable phase adjustment to perfectly time align two sources, and with additional IBP's any number of sources. These were our "secret weapons". With these, our previously very good multi mic sounds were now outstanding.
The original full size IBP boxes were much more than phase alignment tools, being superb direct boxes, reampers, etc. and were amazing tone shapers. Very pricey also but results were worth the coin. Little Labs later introduced the IBP Jr, at a lower price and fewer features (which weren't always needed), so we bought some to add capabilities to our setup. If of interest, scroll down and check out the video at this link, which shows use of the Universal Audio plugin version of the box....
https://www.littlelabs.com/products/ibp (BTW... Just saw the UA plugin price is now $49, down from $99, for those of you into the UA environment)
Radial Engineering came out with their Phazer box not too long ago. Have yet to try one, but seems comparable to the IBP Jr.
Thought I'd throw this out as it relates to combining multi source audio...
Lance