How to expose intranet hosted web services for internet zone to use and vice versa

Our enterprise network has 3 tier.Web,app and db tiers.We also separate our internet and intranet servers into internet and intranet zones respectively.
For intranet web services,our applications talk at the app server layer
We do not go through web layer for web services to talk
Likewise for internet web services,our applications talk at the app server layer
cross zone talking of web services are not allowed.(i.e. internet servers web services cannot talk to intranet servers web services)

Therefore the various ways our web services talk are as follows:
X web services in intranet app server talks to Y web services in intranet app server
X web services in internet app server talks to Y web services in internet app server
X web services in intranet app server cannot talk to Y web services in internet app server

Question:
If I want X web services in intranet app server to talk to Y web services in internet app servers,if I don’t want to use a service bus,what workaround can I do to talk ?