Hi Dhinmar,
Short Answer:
The "(from Field)" option only can pull the value from one field, not more than one. What you are doing requires replacing the values in the text you provide and I have been using replace() to do that sort of thing.
Details part 1
I put the code you supply in your question into a Generate Data node. Then I used a Transform node to create the header field value with the following code:
out1 += in1
out1.myheader = in1.client_id + "\n" + in1.client_secret
Then in the HTTP node you can use "Header (from Field)" like this:

Detail part 2:
I have been following the following pattern a lot lately to do what I think you are wanting to be able to do:
Use a "Input Static" node that defines the structure of the data you want in the output. It has place holders for the data that comes from the input.

The Transform node simply uses replace() to plug the input data into the form like this:

The resulting output is:

Ernest
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Ernest Jones
Precisely Software Inc.
PEARL RIVER NY
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