Web service in Rails to handle two databases

I asked in a question LINK how it was the best way to manage two databases MySQL for a Rails Web App. I searched "web services" in internet and i found a lot of things. My questions are:

1) I have to program an app in Rails, is it usefull to create the web service for this app in Rails too?

2) I have to use two databases , in my Web service how can i have to manage this?

for example, i need the list of the users of the DB1, so i write:

http://localhost:3000/users/

but how do i have to program the web service to be able to separate the two databases? I mean , if i need the users od the DB2 i would use an url like the before one:

http://localhost:3000/users/     (but for DB2)

3) I know that in a web service we should create the WSDL file. But is really necessary in my case? i will not publish the web service so , i think is not necessary to create the WSDL. is it?

Sorry for the newbie questions but it's my first time in web service programming.