An instance template with more than one NIC obviously can't be created in the GCP web console. But it could easily be achieved using the gcloud
CLI with consecutive --network-interface
options.
gcloud compute --project=YOUR_PROJECT instance-templates create multi-nic-vm-template --machine-type=e2-micro --network-interface=subnet=projects/YOUR_PROJECT/regions/europe-west3/subnetworks/my-subnet-1,no-address --network-interface=subnet=projects/YOUR_PROJECT/regions/europe-west3/subnetworks/my-subnet-2,no-address --maintenance-policy=MIGRATE --image=my-image-1 --image-project=YOUR_PROJECT --boot-disk-size=10GB --boot-disk-type=pd-standard --boot-disk-device-name=instance-template-1 --no-shielded-secure-boot --shielded-vtpm --shielded-integrity-monitoring --reservation-affinity=any --tags=allow-health-check
In this example I specified two subnets that belong to different VPC's.