Test 2: 80 MHz - Both Channel 36
The first test established a baseline with both routers set to 80 MHz bandwidth and set to Channel 36 to see how two normal neighboring AC networks play together. In all plots, AP1 is the ASUS and AP2 the NETGEAR.
Two 80 MHz networks - Both Ch 36 - Downlink
AP1 AVG= 254 Mbps AP2 AVG = 348 Mbps
Downlink (AP to STA) shows the ASUS network averaging slightly under 20% lower throughput vs. the NETGEAR for the run. For uplink, the difference in average throughput is around 2x higher than downlink, just shy of 40%
Conclusion: Even for two AC networks, bandwidth may not be shared equally.
Two 80 MHz networks - Both Ch 36 - Uplink
AP1 AVG= 244 Mbps AP2 AVG = 398 Mbps
Test 3: 80 MHz - AP1 to Ch 40
For the next test, I just changed the ASUS' channel selector from 36 to 40 to gauge the effect (if any) of changing the primary channel. Note that 80 MHz wide channels were still set for borh routers.
Two 80 MHz networks - AP1 Ch 40 - Downlink
The downlink plot clearly shows something different afoot, with ASUS' throughput falling to zero about a minute into the run, then everything stopping around 75 seconds.
Pal telemetry shows the Pal associated with the ASUS started scanning for a new connection around 70 seconds and never successfully reassociated. Meanwhile the NETGEAR-connected Pal remained associated. The run stopped when the ASUS' iperf3 connection terminated. I ran this test a few times, with similar results.
Uplink behavior was better, but the ASUS network's average throughput was again lower than the NETGEAR's (~ 35%). So why the downlink wonkiness?
Two 80 MHz networks - AP1 Ch 40 - Uplink
AP1 AVG= 259 Mbps AP2 AVG = 397 Mbps
One of the "features" of at least 802.11ac consumer routers and Wi-Fi systems is how they mislead users into thinking there are more non-interfering channels than there really are.
Contrary to what your router settings may show, there are not really eight (9 if you count Channel 165) 80 MHz wide channels available in the 5 GHz band (I'm using U.S. rules for this example). Although the channels themselves do not overlap, 80 MHz channel bandwidth requires using four of the (20 MHz wide) channels you see in your router's 5 GHz channel selector, for each channel you select.
So there are really only two 5 GHz "channels" where bits flying through the air using one don't collide with those using the other. The channel use diagram below shows this more clearly, showing only one channel (42) in the 5 GHz "low" UNII-1 band and one (155) in the UNII-3 "high" band.
Image credit: Ruckus Wireless
The diagram also shows why adding DFS support, i.e. access to UNII-2 and UNII-2e frequencies, is so valuable. It brings four more truly non-overlapping 80 MHz wide channels into play (channels 58, 106, 122 and 138).
So when you set a 5 GHz channel on a consumer router, you're really just setting the primary operating channel. This channel is where beacons and other channel management frames are sent. The primary channel also determines how an AP adjusts its channel width when it detects a neighboring network using a different primary channel that falls in its extension channels.
This channel width adjustment is supposed to operate on a frame-by-frame basis to ensure the best use of airtime. This Revolution WiFi post is the best explanation I can find of how 11ac's channel use scheme works, and you may still come away confused.
All this is a long way round to attempt to explain why the ASUS router had such a difficult time when I just changed its primary channel. Although both routers saw another network operating on a different primary channel than their own, the ASUS appears to have been more affected than the NETGEAR.
Conclusion: Normal (80 MHz wide channels) neighboring 11ac networks can significantly affect each other.