This discussion is old but i'll share for anyone who wants to know.

I don't think this is possible with the method you have described - i think the waypoint group is either for mob pathing or simply useless code. I have a feeling that the maps themselves partially control mob locations. There is only one workaround that i know:

For example, lets use 'The Crossing' map and you only want the orcs to charge from the left rift.

Step 1) See the file 'TheCrossingRight'? Paste a copy into your mod so the game uses it and simply delete all enemy 'groups' (Group Count) and entity count lines from the file. Now the door will open but no orcs will appear. Just to be safe, i add 1 enemy at the start in case the game doesn't like having nothing to spawn.

Step 2) You're done technically, but its perfectly easy to copy over enemy counts and such from TheCrossingRight into TheCrossingLeft if you want the left side's difficulty to compensate for the loss of the second rift spawns.

Works, thanks. You will be mentioned in my mods. Good day!