[scribus] Spot colors and grayscale images - separation issue

[Victor Papp]

Hi

Given:

1. Two spot colors, say myblue and myred.
2. A one-page document that consists of texts, vector graphics that
uses ONLY these 2 SPOT colors.
3. A grayscale tif placed on the page.

To be done:

I need create 2 postscript plates (will be used on offset press) and
will print with pantone inks, NOT by process.

The problem:

When printing separations, the grayscale tiff is separated into black plate,
I would like to see it on myblue plate together with texts and vector
graphics that uses myblue color.

Question: how to do it?

I found a workaround: removed myblue color and replaced it with black:
it separated then correctly. (Scribus do not know what ink ill be
filled into the press).
But its not an elegant solution and cannot be used, say if i will have
2 tiffs - one should go to myblue plate and one to myred plate.

Any solutions?

Thanks in advance,
Victor.