Can a ZigBee device like the Sonoff ZBMINI be controlled by both eWeLink via WiFi Sonoff ZB Bridge & Home Assistant via USB Sonoff ZB Dongle? The WiFi ZB Bridge doesn’t have LAN mode & as soon as the internet goes down, it also stops working. I want to use the USB ZB Dongle connected to my HA as a backup ZB Coordinator in case my internet goes down.
Actually the location at which I live is riddled with internet outage & thats why I really loved using the LAN feature of Sonoff devices. Later I setup a Home Assistant server on a Pi & recently I ordered & received the USB ZB Dongle. I want to use eWeLink as my primary control method & have Home Assistant as backup. But the issue is I can’t control from HA if my internet goes down. Can you please tell me if it is at all possible to control a ZB Sonoff device paired via eWeLink (using the WiFi Sonoff ZB Bridge) from my Home Assistant (using the Sonoff ZB USB Dongle)?
Short answer : no. A Zigbee devices can only be controlled by 1 coordinator.
Is the long answer yes by any chance…?
It is not.
The long answer would be that each zigbee coordinator/bridge creates a specific zigbee network, and a given zigbee device can only belong to a single zigbee network at a time.
Think VLAN in network terms, but without routing capabilities.
Okay. So right now the only hope is to wait for Sonoff to release a LAN supported ZB Bridge.
Bottom-line: Never buy cloud-dependent devices 
I agree with your point. Do you know of any ZigBee Bridge that works both in offline mode (LAN) as well as online mode. I am mainly going to control it via Alexa Voice Control & in case the power goes out I can control it from my HA. BTW I have an Echo 4th Gen which has inbuilt ZB hub & I have heard that it works in offline mode too. But I cant access the ZB hub of my Echo from my HA. It would be nice if there was a ZB Bridge that can be controlled by Alexa as well as HA. BTW I have tried to enable the cloud facility of the HA (not the subcription), but I can’t get it to work. I mean I followed all the instruction. Not no matter what I couldn’t access my HA from the Internet.
It sounds like you might be better creating a new post specifically asking for help getting your HA cloud connection working. Alternatively, have you considered the subscription?
Doing that now