Can the xbox wireless adapter support multiple devices simultaneously

it can support up to eight devices at a time that are compatible with the Xbox Wi-Fi direct system

Thank you!

As I see it, It could be several factors involved.

The first thing that caught my eye is this: Cisco Model DPC3825.
Since this device is not only a modem, but also a router/NAT built-in with wireless, you will have multiple layers of NAT on your network. That is, if its not already, operating in
bridge mode. Otherwise, you have to configure such operating mode for this device so be set up as a standard modem.

Now for your next device (Linksys E3000). Since this device is a dual-band wireless router, (2.4 and 5 Ghz) make sure 2.4 Ghz band is enabled, since the Xbox 360 Slim's internal wireless network adapter can only communicate on this frequency. If you have both
bands enabled, operating simultaneous, make sure you've selected different network names (SSID) for each band to avoid confusion when connecting a wireless device to your wireless network.

Multiple Console Support

We have confirmed that this network device is unable to support more than one console with an Open NAT.

This is simply not true. Perhaps on an older version of the firmware, but this is a dual channel router and will support two devices with Open NAT as I frequently play both an XboxOne and an Xbox360 or two XboxOne consoles concurrently, with one wired and
the other wireless and both show Open NATs on their network configuration simultaneously.

To Samuel3201 point and your question about 2.4/5 GHz simultaneous. The xbox 360 will not be connected with 2.4/5 GHz simultaneously. Your router being 2.4/5 GHz simultaneous/dual band just means that it will be able to route traffic on both the 2.4 GHz
and 5 GHz clients at the same time, but not combine them. Think of it as the router having 2 phones and talking to 2 people at the same time as opposed to only have a single phone talking to one person, hanging up and calling another. Also xbox has only released
wireless adapters that are compatible on both 2.4 GHz and 5 GHz using the standards b/g (2.4 GHz) a(5 GHz) on the old wireless adapters and then the new wireless adapters can do those other standards plus n(2.4/5 GHz).

How did you manage to determine the source of wireless interference?

The only, well-known, devices that are using a frequency hopping technique is Bluetooth, and by todays Bluetooth standard, it is less likely that a Bluetooth device will interfere with another 802.11b/g/n device operating in the 2.4 GHz frequency space.

Your Netgear wireless network adapter is not compatible with the Xbox 360, nor does the official wireless network adapter support 802.11ac. The official wireless network adapter give support for 802.11n, which include 2.4 GHz and 5 GHz rf space.