Can you get 18V from two 9V batteries
Yes, two 9 V batteries in series results in one 18 V battery.  You connect the + end of one battery to the - end of the other.  The remaining unconnected battery ends are the ends of the overall 18 V battery.

While you get more voltage, you don't get more current.  If each battery is capable of 9 V at 300 mA, for example, then the combined battery is capable of 18 V at 300 mA.