This type of question can be asked in the "Print Design" section: http://forums.adobe.com/community/design_development/printdesign
This is what I did when I was working in a printshop:
One way that you can do it is by outputting 2 negs with different LPI (The word "Void" will be the lower LPI) but keeping the same halftones for both. Dupe both negs into one. Because the critical step is the alignment, I recommend stripping both negs first.
You have to be very careful when duping the negs, timing will determine the quality of the screen and the preservation of the DPI sharpness.
I recommend a velox before producing the plate. Use the velox to make a copy for testing.
Depending of the copy you can determine if the timing was right or not. I will suggest keeping notes of the timing because every machine and brand of negs produces different results.
This is the way I used to do it. I hope this helps!
Do you have FreeHand at your print shop? It allows you to set object-level halftone settings.