1. Connect both soundcardS with a midi cable. On the master computer, the cable will be connected to the midi out port, on the slave computer to the midi in port.
2. Go to the preferences of ableton live, go to the midi sync tab and find your soundcard in the midi ports section :
3. On the master check sync on the output
4. On the slave check sync on the input
Leave the midi clock on song
5. Check the "EXT" button (on the left of the tap tempo button) on the slave computer.
6. Press play on the master, the slave should follow.
more on this topic shortly;)