If you look at the above pic you will see this...
At hte top of the photo is the regulator that is attached directly to the CO2 tank. The outlet of the regulator has a short connector pipe, then a tee, then another connector, then a second tee and finally a needle valve. The "top" leg of each tee also has a needle valve to which the tupib that goes to the tank is attached. One needle valve controls each tank. The 2 tees and their connectors are the "manifold". Manifolds can be bought or made, as this one has been. Usually the ready made ones are like a 3-5 way tee. Same exact thing just wiht more parts. THe DIY is a littel cheaper but has more room fo rerror(leaks) if you do not do it correctly. Not that it is hard. Some teflon pipe tape and a couple wrenches is all you need. The bottom needle valve in the above pic is not being used at the moment. It is probably either for future tanks or maybe to drain the system. Regardless, if you only have 2 tanks you could just use one tee. One outlet of hte tee comes from the regulator (or selinoid) and the remaining 2 outlets have needle valves attached. You would probably want to use a bubble counter near each tank to fine tune each easier but that all depends on you method of diffusion.
Hope this helps a bit.