If the Alexa skill is available in your market (as it is in the US and many other countries), the recommendation is to use that instead of the Echo app. The Echo app is older and makes the hub emulate a Hue Bridge so Alexa can control it (since it natively supports Hue Bridges), but the Alexa skill is more powerful and supports more kinds of devices.
As far as I know, the Alexa skill supports only one hub, consistent with what you see. If you have devices from multiple hubs that you want Alexa to control, I think what most people do in that situation is use one of the "hub device sharing" apps to get devices from one hub to another, in your case to the hub with the Alexa skill. Hub Link is built-in and can do this, but HubConnect is a popular community alternative that is more powerful (supports more devices and custom devices if needed) but will require installing some custom code.
HubConnect can work through either LAN or cloud, so if they're completely separate LANs, at least the latter option should still work. I haven't tried Hub Link with any such configuration, but I don't think that will work (unless maybe they're vLANs and you can "see" one from the other).