This piece below is the exact same as this one you listed, just with different percs. Many people said the follow about the piece I list below, which is similar to yours.
http://www.dhgate.com/store/product/two-function-blue-color-glass-bongs-water/199022739.html
The percs are too high and the can volume is too small for those style percs.
Most of the water in the bottom can will get sucked up into the top can because the bottom can is even smaller than the top.
Water level will be tricky. You will have to keep the water level right in line with the perc or even slightly below the top of the perc to avoid splash back. Sucking very lightly will also help avoid splash back and sucking water up from the bottom can to the top can.
The top perc in each can will only be the one firing unless you pull harder but like I said above you will probably need to pull very lightly.
I hope the slightly different percs on yours work better! Worst case if you don't like it just sell it and get your money back (or make a little profit) and then buy a new cheap one or two
Let us know how you like it after you get it! Vids would be great too