There are complicated employment laws which need to be adhered to
correct, and most of the posts on here indicate a complete lack of understanding of what those laws are. Also most people also don't seem to understand that these issues are not black and white either.
As the companies are being merged together
nope, they aren't. The parent company of one company is buying the other company who would otherwise go bust within the month.
the law requires the employees DoJ their original company to be respected as continuous service.
it probably does require that, and if it does I'm sure it will be respected where it needs to be, for statutary issues such as pensions, maternity, redundancy etc.
the rest of the stuff such as Seniority lists, contractural t's and c's, salary scales have nothing to do with DoJ and will be negotiated by both CC's.