What do you mean by "one application in both platforms"? Is it the same application written in two different languages? If it is, why on earth would you wanted do that? I mean, besides of duplicate you work, and not only for development, but for testing, maintenance, etc... I'm sure you have your reasons to try to do this, but can you share them with us so we can properly evaluate the scenario, please?
I mean one application with different platforms means one module in rails and another module in php or phython or perl like that
I really don't see how doing this can possibly help you. You might have a pretty particular scenario to be considering this, I think. I wouldn't mix the platforms. Reasons? It's harder to maintain, it's harder to setup, it's harder to develop, it's error prone, might lead to bad practices on one or another platform, and the most important of all is that I can't see even one benefit of doing this.
But your application should be designed based on your needs and I don't know your needs. So, if you don't have REALLY SPECIFIC requirements, I'd STRONGLY recommend you not to do it.
Hope it helps.
The only reason I would see for doing something like this is if there were something that one of the platforms offers that the other one does not have or if doing something in one of the platforms is so much easier that it would be a waste of time trying to duplicate it in the other one.
My $0.02.