>Because of this need for ongoing communication between the two applications
>I guess there might be a completely different way of approaching this - thanks in advance for your suggestions.
>

You want more than "Inter-application communications", you want two large applications written in two separate environments to run as one. I would try to talk with my users and get exactly what is necessary/used for each system from the other. Then try to design a plan for integration.

You may find that the users only need a very small amount of information from the other application, and you can create a form/whatever in the existing application that shows that information. Ie, looking up a customer and seeing what products he has purchased in your sales application from your accounting application.

As developers, we always attempt to code a solution when it might not be necessary. I hope this is the case here.