12v Water Pump

*As per your question , you want to run the pump in parallel.
So, you will get double discharge if your piping design is suitable for parallel operation.
Flows Q1 and Q2 combine, and that is what flows through the pipe :
Qpipe = Q1+Q2
*If you want to increase the head pressure , then you have to run the pumps in series.
*You will not give more head pressure if you run in parallel.
*YOU must go through the following links for having clear idea about "pumps in series and pumps in parallel:

http://www.ceet.niu.edu/faculty/kostic/Design-Methods-Ch4-pumps_452/sld011.htm

*Following is a very good link,please visit:

http://www.pumpingmachinery.com/pump_magazine/pump_articles/article_13/article_13.htm