As I understand it, PDF files cannot have gradients from one ppot colour to another spot colour. Using iTextSharp, if I try this, it results in an exception when the spot colours on the gradient stops do not match.
However, Adobe Illustator CS6 is able to create a PDF file which includes a linear gradient from one spot colour to a different spot colour.
Looking at the resulting PDF file, I see the following:
13 0 obj
<</Colorants 15 0 R/Subtype/NChannel>>
endobj
15 0 obj
<</Spot#20Blue 16 0 R/Spot#20Red 17 0 R>>
endobj
16 0 obj
[/Separation/Spot#20Blue/DeviceRGB<</C0[1.0 1.0 1.0]/C1[0.0 0.0 1.0]/Domain[0 1]/FunctionType 2/N 1.0/Range[0.0 1.0 0.0 1.0 0.0 1.0]>>]
endobj
17 0 obj
[/Separation/Spot#20Red/DeviceCMYK<</C0[0.0 0.0 0.0 0.0]/C1[0.0 0.993988 1.0 0.0]/Domain[0 1]/FunctionType 2/N 1.0/Range[0.0 1.0 0.0 1.0 0.0 1.0 0.0 1.0]>>]
endobj
9 0 obj
<</Color[20224 32768 65535]/Dimmed false/Editable true/Preview true/Printed true/Title(Layer 1)/Visible true>>
endobj
Based on the above, it looks like a multi-spot-colour gradient can be accomplished using a "Colorants" dictionary.
Does anyone know how to do this using iTextSharp, or iText (java) ? I cannot seem to find references to Colorants in the iTextSharp source code.