Most controllers will run 2 valves off the same station terminal. Are you sure that your running two valves simultaneously wont compromise the system's original designed hydraulics? It seems to me that you would more than double the flow in the main creating velocity/water hammer issues.

I am not using it for irrigating lawn. I am using it to
do water change in the fish room. Since I will use more
than 2 valves per zone, I have decided to either use a
power relay or power FET to drive multiple valves. This
way I don't have to worry about the voltage drop coming
from the controller. Also, I don't have to be particular
about which controller I need to get. I could use an old
laptop via the RS232 port as the controller but then I am
at the mercy of Bill Gate's operating system.

Most irrigation controllers will run 2 or more but in some cases you will burn out the station on the controller.

Here is a link that might be useful: diy sit

There are commercial controllers out there that will run, three, four or six at a time. The key is having a transformer that has the capacity to do this, and having a controller that can be programmed to do this. Hunter, Rainbird and Toro all make controllers that will run 4 or 6 zones at a time. Any more than that, and you will have to spend over $1500 to get into a controller that will do it.

There are two primary things to consider. Can the controller handle the number of valves you want to run, and can the transformer handle the number of valves you want to run.

A typical irrigation valve is going to require about 350mA of current to open the valve, and about 250mA to keep the valve open. If you have a transformer that can only supply 1A of current, you are already pushing the limits if you try to run three valves at once, regardless of the electrical specs of the controler.

The next consideration is the electrical specs for the controller. Some controllers can handle enough current to run two valves off of the same circuit. Some controllers can handle enough current to run two valves, but only if the valves are on different circuits (i.e. there can be different limitations to the controller as a whole and different limitations to each circuit).

The "cleanest" solution might be to use a cheap controler (i.e. one that can not supply much current), but have the controller only control a power relay and connect all the valves to the relay. Then your only limitation is the limits of the relay and the power source.

funny... I wanna do it the other way around. 1 valve on 12 zones to get more timer options haha

I am running 16 valves one at a time. Using a centre tap transformer, diodes, mains 15 minute timers outputting to 12 transformers to run relays and one electronic cyclic time to on off whole system to accurately control watering time for each valve.

Remember diodes conduct in only one direction and the transformers can switch on and/off relays to give different signal flows. On the return route use two wire from batches of solenoids and use a spdt switch to select the batch connected in.

Think about it and it will all come to you and you will get to 16.