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I think you may have to keep them separate for now. Maybe find a way to pass that info onto your users, that if they want to do an image seach, due to server limitations (just for technical merit) you are unable to do both and that they should go to the gallery home page to do a search. All other searches can be used normally. Throw it in an FAQ and pin it into your forums.
Anyhow, I personnaly am not upgradding my xoopsgallery until the actual gallery has been upgraded along with the next major XOOPS upgrade. Following that, the xoopsgallery mod makers will need to make their upgrades compatible with both. I'm using the 'if it ain't broke, don't fix it' rule since I have everything I need functioning perfectly on my site as it is, without hacks or modifications. (other then esthetics and inconsequential changes)
I know it's not much help, but I get the impression that what your looking for isn't detrimental to your sites operation (for the moment).
Up to you though