Do you have ancestos for both? If so, it is a little complicated.

I am speaking from memory as I am away from my home PC so please test first! When I merged in error, this is what I recall doing:

1. Create unrelated individual for one individual (A), using a different DOB.
2. Remove inncorrect spouse (C) from second individual (B). You should get the option for the children to accompany C. Do so.
3. Marry A & C
4. C should now have two marriages: A and "Unknown spouse"
5. Enter exect details of A (dates, name, etc.) under unknown spouse.
6. You should get a merge prompt. Otherwise Merge
7. Add A to correct parents, if known.

Good luck
Bruce