A: Software limitation
Instead of using the 'watermark' tools, you could simply open the relevant header and use Insert>Picture, Insert>SmartArt or whatever to insert whatever you want to use for the odd/even pages. You might have to fiddle around with the positioning a bit, but you'll end up with two independent watermarks.
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Cheers,
Paul Edstein
[Fmr MS MVP - Word]
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