I'm assuming you mean printing like this:
1st sheet side one contains pages 1 and 2 side by side
1st sheet side two contains pages 3 and 4 side by side
2nd sheet side one contains pages 5 and 6 side by side
2nd sheet side two contains pages 7 and 8 side by side
and so on.
Am I right?
I'm also assuming that you want to limit the printing to 30 SHEETS of paper, which given double-sided printing and two pages per side sheet would be equivalent to 120 pages of text. This means that you will still need to do some serious editing in order to reduce the size of the manuscript as with its current length you would use 84 sheets of paper. Changing the font and margins will not reduce the text by such a large amount. You will save a few pages, but not over 200. I've written enough papers to know.
Next question: are you simply wanting to PRINT it this way, or do you want to COMPOSE it this way. The answer to this question will determine WHERE to adjust the settings: The choices are with the printing menu options alone, or a combination of document settings and the printing menu options.
The first will simply shrink the size of two of your pages to fit on one sheet, in which case I would not reduce the font to anything smaller than 12 as the entire text will be miniaturized. I'm not quite sure yet what the second will do: I need to experiment to find out, and will need to know what word processing program you are using.
p.s. If this manuscript is to be submitted to someone, I would strongly
urge you to find out if this print format will be acceptable to him/her.