Not sure, what you with multi-home.
When a service is avaiable under multiple IPs in different subnets.
our system admins told us, that when you are in the company's intranet you can reach it ONLY via an internal IP address and NOT the external one
That's madness. You will only run into problems with this setup, as it makes things unnecessarily complex. Even if your company policy is to restrict access to internet IPs, services like your companies XMPP server belong into a DMZ, which is reachable by your internal IPs *and* externally.
I could tell a dozens examples why you will run into problems with that setup. Good luck with that. DNS is your smallest problem, as that's where SRV records are really useful.