Reusing Eclipse run time variable input

Question

I am trying to create an eclipse runtime tool to diff 2 files. For my specific instance the files have the same name but are comparing versions.

I created a runtime external tool configuration using p4merge using these arguments:

gen\${string_prompt:File Name} gen2\${string_prompt:File Name}

What I really want is 1 dialog box where I can use the variable 2ce.

Anyone know if this is possible directly in eclipse other than writing a batch or shell script?