So I have two clusters at the moment (soon to be a few more, once I get this working), ClusterA and ClusterB.

Is it possible for one ingress to interface with services from both clusters?

ClusterA hosts the front end and the ingress, while ClusterB hosts the back end.

The excerpted ingress is below. Everything bar the back end works.

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  annotations:{...}  
  selfLink: /apis/extensions/v1beta1/namespaces/default/ingresses/test-frontend-ingress
  uid: //
  spec:
  backend:
    serviceName: idu-frontend-XYZ
    servicePort: 80
  rules:
  - http:
      paths:
      - backend:
          serviceName: test-backend-app-service
          servicePort: 8080
        path: /api/v2/
      - backend:
          serviceName: idu-frontend-XYZ
          servicePort: 80
        path: /
  tls:
  - secretName: tls-cert-name
status:
  loadBalancer:
    ingress:
    - ip: 123.456.789.012

Back end service URL:
https://console.cloud.google.com/kubernetes/service/asia-southeast1-b/test-backend/default/test-backend-app-service...

URL the ingress tries to point to:
https://console.cloud.google.com/kubernetes/service/asia-southeast1-b/standard-cluster-1/default/test-backend-app-service...

So what I've gathered is the ingress can only interface with things in the same cluster as them? test-backend and standard-cluster-1 are the cluster names, and they are both on the default namespace. Isn't that kind of pointless, as you can only deploy one thing to each cluster? Unless your images contain multiple apps, in which case it isn't really microservices anymore.