Cross-Branch merging in TFS?

I agree with Harpreet that you may want to revisit how you you have setup you branching structure. However you if you really want to perform this type of merge you can through something called a baseless merge. It runs from the tfs command prompt,

Tf merge /baseless <<source path>> <<target path>> /recursive

Additional info about baseless merges can be found here

Also I found this document to be invaluable when constructing our tfs branching structure Microsoft Team Foundation Server Branching Guidance

tf.exe merge /recursive /baseless $/TeamProject/SourceBranch $/TeamProject/TargetBranch

• MSDN: How To: Perform a Baseless Merge in Visual Studio Team Foundation Server