In plan, pull each roof back a bit from center line and then select one and
then while holding down the ctr(pc) (mac opt?) key, put the small mercedes curser over what would be the "high edge" of the other half and click, it will move the high end to the center, then reverse the process.
They should then join perfectly.
If you try to do this manually, it's hard to get it exactly right and you will
most likely have gaps or overlaps.
For more complex questions you should make a profile with machine and software you are using.