I am using Sugar Enterprise edition 126.96.36.199.
I am having two queries
1 . How to receive/consume WSDL provided by other (Siebel) CRM?
2 . How to have a two way communication between both CRM's?
You can create some custom entry point to your CRM.
There are multiple ways of doing this. For example, a logic hook can leverage the NuSOAP and CURL PHP extensions to make calls out to a third party web service to either pull or push data. Likewise, you could use a custom dashlet to do the same. You could also do it in a custom controller for a given view, or a brand new view outright. A custom scheduler job, custom entry point or custom endpoint is yet another possibility.
In short, you have many options, but which is best will be dependent on what you wish to accomplish and the expected client-side behavior.
Retrieving data ...