With 80 ppm tap water reading before ro ,  You will not need an one 11 ltr di vessel never mind two as you will be wasting your money . 
If you are purchasing that 300 GPD  unit you will have all you need for your current and future usage . 
Although most ro units have 3 pre-filters and a 11 inch  di vessel after ro but that set up should do you very well . 
If your TDS is 80ppm after ro unit you will need two 11ltr di vessels or 15.4 ltr from 
www.daqua.co.uk   and if you really dont have a clue on the best setup contact Daqua and they will provide you with the best set up which will be worth spending a little bit more  . Mike