I would be using different subdomains for different services, but those services would reside in different K8S clusters. I would like to know if I can just copy the wildcard certificate across the clusters. Also, if there is a similar example somewhere please link me to it. Any help would be appreciated.
So there is no native handling of multiple clusters in cert-manager.
That said, there is nothing to stop you copying across the resulting 'Secret' resource between clusters, either manually or automatically.
The 'kubed' project (by appscode) has support for syncing Secrets between clusters: https://github.com/appscode/kubed. Full information can be found on their website: https://appscode.com/products/kubed/0.8.0/guides/config-syncer/inter-cluster/
I hope this helps!