[dev]

As super_dan says it depends on the charger design, switch-mode chargers are much more efficient than linear ones. Best bet is test each charger's current draw.

See http://en.wikipedia.org/wiki/Switched-mode_power_supply for an excellent comparison between switch-mode and linear designs.

Generally if your charger can charge more than 8 cells (or about 3S lipo) from 12V (ie it can put out more voltage than its input voltage) then it is a switch-mode design and roughly your peak charger input current would be ( 9.5V x 4A ) / 12V = 3.16A (assuming it was 100% efficient, which it won't be, probably more like 80%, so maybe 3.9A max), where the max battery voltage for the cells is about 9.5V, the charge current is 4A and the charger is running from a 12V supply. Note that this means as you increase the output voltage the input current goes up, so as your battery peaks the current peaks.
I believe all the Pro-Peak chargers (Constellation, Prodigy, Elite) are switch-mode, as is the Much More silent charger, obviously the pulsar ones are and many others will be.

Linear designs, instead of switching the power, dissipate the excess voltage as heat, so if you are charging at 4A, your cells are at 7.2V and your PSU is at 12V the charger is dropping (12V - 7.2V) = 4.8V across itself, the current through the charger is the same as through the batteries, so the power (heat) in the charger in this case is 19.2W, hence why it needs fans to keep it cool! On top of that charge current you have the current to run the LCD, fans, microcontroller and all the other bits in the charger, meaning the overall current is more than your charge current. Note that unlike the switch-mode chargers the current drawn over the charge cycle will be pretty much constant.
A Muchmore Ctx-cell master platinum (the one with the loud fans) charging at 5.5A draws about 6.5A, but they are big fans and a fancy screen!
Chargers like the Pro-Trak and many others are linear; normally they have the cooling fans running most, if not all of the time, to keep the heat down.

I have a 12A supply that runs two switch-mode chargers, charging at 5.5A, without any problems.

If both your chargers were linear then it might be too close to the limit of the psu at 10A, if at least one of them is a switch-mode design then it should be okay.

[This reply has become way more complicated that I ever intended...]