What you are describing is very common in industrial RO units where you have a primary unit and a secondary recovery unit that takes the reject stream from the first membrane. They run at much higher than 65 PSI pressure and typically these basic ones get 75% recovery, meaning 3 gallons of clean permeate per 1 gallon of rejected water. If you do it this way you will not be able to tie the primary and secondary membrane permeate streams together as they will be at much different pressures. You will have to run them separately to your permeate tank. Do you need DI? It all depends on how low of TDS you want in your water. If you want zero you will have to run DI. If you just want low, like 1-10, then RO should be just fine alone.
Low pressure industrial boilers will use RO water as their feedwater without any issue. Once you get up to high pressure super-heated steam boilers for turbines you have to do DI after RO to remove everything, especially the really hard to remove silica minerals that will plate out on your turbine blades.