Yes, index markers can be confusing until you figure out what the marker text is actually saying to FrameMaker!
It sounds like you're looking for something like
Assignment Creation 15
See also Associate Assignment
You need two entries to get this result. It could be two markers, but probably best if you have two entries in the same marker. It looks like you already understand that the semicolon separates entries, and the colon separates levels within an entry.
The trick is to get each entry to be complete on its own.
In your first example above, you have two entries, one under A for Assignment, and one under S for See Associate Assignment.
In the second example, you have one entry, under A, with a second level specified for <$nopage>See Associate Assignment.
Your provided example marker text is close, but needs a little extra info.
Try this entry instead:
Assignment Creation ; <$nopage> Assignment Creation : See Associate Assignment
The first entry is simple, and correct on its own, and now the second entry contains both the first and second level info needed. If I had to guess, the <$nopage> control needs to be at the beginning of an entry to work properly.
Lastly, if you place the <$nopage> control in the first entry, it will most likely turn off numbering for any subsequent entries in this marker.
Let us know if this fixes the issue for you!
-Matt