I raised this issue a year or two ago, and got told they wouldn't fix it. I don't even think the documentation saying it wouldn't merge orders with different currencies was ever fixed.
Fingers crossed a more sensible attitude applies now and it does get resolved.
I have raised addition for extra condition (same currency) to our developers, I would expect it to be released within few weeks. I will keep you posted, docs will be updated accordingly.
Please confirm that we talking about Manual Merging here and not AutoMerge.
For me this is manual merge on the desktop version of LinnWorks.
I've just unknowingly been able to merge two customer orders (same customer, same address details etc).....the problem is they are in different currencies...this clearly shouldn't be allowed to happen, so Linnworks need to make their 'Merge Orders' code more robust (to ensure orders placed in diffferent currencies can't be merged)