Determine trigger origin

Yup you are right

Doing some expressions evaluation and hidden checkboxes you may reach what you want, but won't be easy neither to explain here I'm sorry.

What you are trying to accomplish? maybe another way.

Ah, you're right -- you can't double bind the parameter with both context ids... in my head I was thinking you could pass it like a message with the clicked event of the button.

This may be a little wonky/messy, but one way you can do it is:

1. Create a session variable called contextId
2. Add SetGlobalSessionStringValue from CurrentSessionInfo to the mashup twice
3. Tie each button clicked event to each of the separate SetGlobalSessionStringValue with each contextId as the value param, and set the name param to context id, and each button clicked event to trigger setting the session variable
4. Add GetGlobalSessionValues service from CurrentSessionInfo and have it run on the service invoke complete event from the two SetGlobalSessionStringValue services
5. Put the result contextId  from the GetGlobalSessionValues as the input parameter of the main service you want to call, and call it on ServiceInvokeComplete of the GetGlobalSessionValues.

Then continue as I had posted above. This same result might be easier the way you suggested earlier, however.

Edit: Actually, you could simplify this on the mashup end  by calling the GetGlobalSessionValues  in the main service you want to call, instead of on the mashup. This way you could just bind ServiceInvokeComplete of each of the SetGlobalSession... services to the main service you want to call, and get the current contextId in that service before doing your if or switch statement to see which one is the current clicked property. Note that I wouldn't use the contextId from the session tab in the mashup, because it may not get updated when you call the  SetGlobalSession.. services.