On my DTXplorer kit I have an AUX in, and I will make the assumption yours has the same thing.
If it were me, (and I just tried this on my DTXplorer so I know it works) I would run the Laptop into the DTX, adjust the balance using the AUX IN volume (Laptop volume) and the DTX volume. Run a 1/4" TRS to 1/4" mono TS splitter cable from the Left and Right outputs on your DTX to a Behringer HA4700 headphone amp (or similar, its just what I have).
The two mono ends go in the "Left Out" and "Right Out" of the DTX. The stereo end (TRS or Tip Ting Sleeve) goes into the "Direct In" on the front of the HA4700. Then up to 4 people can listen in and have their own volume bass and treble controls.