Hi,
if your clusters are new enough you could use the experimental remote migration feature [1].
The most stable and well tested approach is probably creating a backup and restoring it on the other cluster [2]. All you need for that is basically some storage that you can either transfer between the two clusters or that they share anyway. For example, storage server that has two NFS shares one each for a cluster, create the backups on cluster A, configure cluster A's backup storage on cluster B, restore the VM on cluster B.
[1]:
https://pve.proxmox.com/pve-docs/api-viewer/index.html#/nodes/{node}/qemu/{vmid}/remote_migrate
[2]:
https://pve.proxmox.com/pve-docs/chapter-vzdump.html