Merging two (or more) partitions isn't by definition reversible as it is more than adding up the space occupied by the partitions involved.
You're not merging partitions essentially, but you're merging file systems. Depending on how far the merge went before it crashed you may be dealing with a highly inconsistent file system.
If we for example consider the merge of two NTFS file system we'd start with two master file tables (MFT) but they're both referencing clusters from different offsets. If we add a partition move into the mix, both will be working with wrong offsets. The tool doing the merge will be basically merging the 2 MFT's into a consistent one, if this gets interrupted at some point you'd end up with a mess.
Failed or interrupted partition merges can be among the hardest logical data recovery cases, much harder than deleted or formatted volumes for example but it depends at what stage the merge fails. At the start of the operation and at the end, we have more of less consistent file systems, that should be less hard to recover data from.
My weapon of choice for such cases is DMDE.
If you need the data I suggest at this point:
- You stop trying to fix this 'in-place' as it may make the situation worse
- If you insist on fixing first create a sector by sector disk clone
- Instead try read-only file recovery
File recovery would involve using a competent file recovery tool to scan for candidate file systems (not partitions). Below is just an example of an in progress scan. Examine each of the candidate file system by selecting and Open Volume option.
If file system looks messy like so
Click 'All found / Virtual FS', the pure FS system reconstruction
Now locating and try previewing some larger JPEG or BMP files. If these correctly preview, you have a consistent file system, copy everything you need to a different drive.