mainly here you need to write new test cases which are care for data base integration between the two modules, and you have to notice if any changes made on "module 1 " could affect " module 2".
the most important thing to notice here is the data cycle, BIG QUESTION you should know its answer " does data from module 1 used in module 2 or the other way around? and if it must, does is it stored correctly?"
that's my point of view for merging, hope it was helpful.