Merge sets of tasks together?

[Bob]

I created a new worksheet and listed two sets of tasks. I made each set a
different color. I want to freeze their dates and then merge them together
on one time line. Can you tell me the easier way to do this?

 

[Steve House]

What do you mean "worksheet?" Project doesn't deal with worksheets. Are
you saying you have a project file with two summary tasks, each of which has
a number of child subtasks indented under it? What are trying to do when
you say you want to "merge them?" BTW, it's very unlikely you can "freeze
dates" since Project is fundamentally a date calculator, moving tasks around
in such a way as to alter the parent child and predecessor successor
relationships is bound to change the dates since it is those relationships
that partially determine the dates in the first place.