Can you use an attribute value from a sibling?

I'm trying to pull an attribute value from a sibling element and it's not working. Here's what I've got:

<Figure Fignbr = "8">

    <Sheet>

        <Image Sheetnbr = "4">

        <Title> Figure Title </Title>

        <FigNumber>

    </Sheet>

    <Sheet>...

My EDD for the FigNumber element is:

Suffix: Figure <$attribute[Fignbr:Figure]> (Sheet <$attribute[Sheetnbr:Image]>)

Which should (in theory) look like this:

Figure 8 (Sheet 4)

An obvious fix would be to move the attrubute from Image to Sheet, but I'm working with current files, not creating new ones, so I'm trying to avoid changing the structure. I'm using FM10.

Thanks!

ebpdx