Get the angle and distance between 2 points, then do a polar offset from point 1 towards point 2 with the angle you got and random distance between 0 and the distance between the points.
If you want a minimum distance to both points you can easily check the random value of distance and get a new value if its not right or:
1) substract 2 * mindistance from distance between points
2) get random value between 0 and new distance between points
3) add mindistance
Just create a demo map where you move one unit directly with abilities, while at the same time a trigger catches the key presses and moves a second unit parallel to the first. Then add some slots and upload it on bnet, so everyone can test and compare the different systems effectively.
Bonuspoints for a video of said map with multiple players testing at the same time.
I extrated the function list for the native library from the latest patch: Versions\Base15343\patch.SC2Archive Mods\Core.SC2Mod\Base.SC2Data\TriggerLibs\nativeLib.galaxy.
Its all the non-native functions we have availible in the editor. Handy as reference for custom code.