I've seen folks "underlap" their crossover regions where subs stopped at 90HZ, tops started at 100Hz, and I've seen folks "overlap" this same region subs at 120HZ, tops down to 100HZ. It all depends on the summed output at the crossover region.
One thing that most people miss which helps with a two way setup is the timing between the top and bottom boxes.
Crossover slopes, box design, and cabinet alignment all play a part in lining up the full range of sound hitting you in the back of the room.
The first thing to try is run a tone at the crossover freq through the top box, then the bottom box at the same SPL.
Then play the tone through both at the same power level. You should have (theoretically) a +6db gain in output.