Change Printers Default Single Sided Setting #mac

 Well, there is an easy way...

http://localhost:631/

This prompted me to do
cupsctl WebInterface=yes
to the terminal... which enabled opening up the printer options... then I could set up the duplex printing.
Went to
http://localhost:631/admin/#General
Then, I had to select "Two-sided, Long-edge binding".

This enabled printing on both sides.

Print a large poster on multiple A4/paper

There are two ways:
1. Save the (powerpoint) document as  pdf.
2. Open the pdf, print.
3. Select the appropriate options in the poster option in the page size and handling section
4. The options are Tile scale: 200% for example, overlap by few tenth of the inches
5. Cutmarks and labels etc are optional #4 does the magic!

Another way that I learned from the internet is to paste the picture in the excel sheet. The excel is smart enough to segment the pic into multiple pages. And, I used to think that extra flowing line being printed into second page was a bad feature in excel! :)