As long as you wire them in parallel they will work, if you try to wire them in series you will double the voltage.
Use your multimeter to find out which is positive on each transformer and connect them together. Then connect the fan to the leads.
http://www.4physics.com/phy_demo/batteries/BatteriesInParallel.html <<--thats in relation to batteries, but is simple, easy to understand, and has pics.
I would probably buy a surge/overload protector power board (shouldnt be expensive) and have both power packs plugged into it. Just remember, $25 for an overload protector (at the most!!!) but how much would it cost to rebuild your grow room from fire damage at the least, or your whole house at most.
The car battery charger is not a horrible idea, and you can get buffered and protected chargers...but they are usually the deluxe models..It will state overload protection on the box if yours has it.
Also, theres a thread on this forum thats all about turning a power supply for a computer into a 12v benchtop dc regulated supply. A higher gade PSU will have a higher rating, thats only a 100w one.
Think of voltage like your house water piping, and amps are like the pressure behind it...If you try to suck more amps through the wire than the voltage can provide the resistance in the wire will make it glow white hot, this only takes half a second and its a real danger! Id say either go with the 2x plug packs, or look into buying a better pc supply. PC supplies are great...VERY filtered, very stable, good protection...
If you still need any help let me know, send me a pm or ill subscribe here and see how you go. I always get myself into trouble trying to save money and do stuff myself lol...luckily I am studying electrotechnology at the moment..Ive not had a fire yet...well once on my motorbike when the shop didnt secure my battery properly and the positive post hit the frame i melted all the ground wires lol...but that wasnt my fault.