Well you could use the h-bridge and an external 12v supply and tie the grounds together. I'd only do this if the Siemens inputs are isolated from any AC.
If they are not then I'd suggest either an optoisolator driving the H-bridge or just get a relay and use one pair of contacts for +12 and one pair for -12.
It's very easy to drive a relay with the arduino and it will work whether the Siemens is isolated or not as it provides the isolation. A program to run it would be trivial to write.