You have to set one to external sync. I cant quite remember where the menu is but it should have 3 options, INT, REM, EXT. Change it from INT to EXT, this 303 will be the 'slave'. Hook it's midi IN to another 303's midi OUT and set that 303 to INT (which it should already be) Hit play on that 303 (the 'master') and it should control the 'slaves' start, stop, and tempo controls. Its all in the manual if you have one, good luck!