I wrote a tutorial for merging text in AI, you can find it here:
Essentially, some fonts, cursive ones especially, overlap parts of the individual letters. This isn’t an issue when using a desktop printer (black + black = black), but on a cnc-type machine like the Glowforge it can cause those overlapped spots to be engraved twice (engrave + engrave = double engrave) , or it can cause voids (engrave - engrave = no engrave). Best practice is to avoid the overlaps altogether by using the Unite tool in AI.
(signage-specific design software are the only programs that I have used that have a “weld overlapping text” option in the save dialogue; if you send overlaps like that to a vinyl cutter it will makes cuts that you really don’t want.)
-edit- … I see you already figured out what I meant!