When you say App does it ment two Salesforce accounts or two salesforce applications within a single Salesforce Account?
If you wish to integrate two Salesforce orgs, you can use the Salesforce APIs to accomplish this: https://help.salesforce.com/HTViewHelpDoc?id=integrate_what_is_api.htm&language=en_US
If you wish to integrate two Salesforce orgs, you can use the Salesforce APIs to accomplish this:
https://help.salesforce.com/HTViewHelpDoc?id=integrate_what_is_api.htm&language=en_US
REST :https://www.salesforce.com/us/developer/docs/api_rest/
SOAP:http://www.salesforce.com/us/developer/docs/api/