Combining maps into a single map

[DuckReconMajor]

What is the easiest/cleanest way to combine two maps? All I can think of is:

 

1. Open two instances of GZDoom Builder
2. select all sectors/things (if possible) of one map
3. copy and paste into the other

[Fonze]

Additionally, since db2 that family of editors has had options you can use to determine the behavior of stuff like conflicting tags, etc.

[Nine Inch Heels]

Sounds about right, yeah.

[DuckReconMajor]

Thank you both. I'll mark Fonze's answer just to show it's been answered.