I'm trying to create a load balancer and attach two virtual networks in order to have them in different zones(HA). I encountered the following error message when I try to add a second virtual network in the Backend pool section:
A backend pool can only contain resources from one virtual network.
Is there any possibility to do that?
No, it is not possible to attach multiple virtual networks to a basic tier Azure load balancer - which I assume you are using. To achieve your desired infrastructure as described above, you would need to use a standard tier load balancer which supports availability zones and offers a 99.99% SLA (Service Level Agreement).