Hello: If Zentyal has in DNS Module the DNS yourdomai.tld, then it does not matter how many forwarders you add, I'm afraid. Zentyal will be authoritative for that domain and will not query the forwarders for queries regarding such a domain.
If you need such an environment, then I would suggest you to copy any needed record from your ISP domain(MX A CNAME; SRV, TXT) to Zentyal Domain. Altarnatively, you may make zentyal to use a different domain