Here's a thought I just had. You could attach a line to a chute in the booster, then hook a peg on the other end. You could then arrange a hole in the lower end of the sustainer (just be careful not to provide an alternate escape for the sustainer's ejection charge). The peg would be inserted in the hole when preparing the rocket for flight. As the booster is pushed away by the sustainer's ignition, the line would cause the booster's chute to deploy, then the peg would be pulled out of it's hole.
This would be tricky in three ways though.
One: The peg tightness would have to be just so. It would have to be tight enough to get the booster chute pulled out but loose enough to pop out once the chute's free.
Two: The line and the chute would be in a perfect position to get scorched. That could probably be handled be creative positioning of the motor mount tube and the coupler.
Three: The tug of the plug could cause a "tip-off" effect. I.e., the sustainer could be turned a bit in its flight at the point of plug separation. This might be offset by using more than one plug or by using three lines that end in a sleeve (rather than the plug) which encircles the end of the sustainer's motor mount tube. The sleeve would slide off in a way that would be much like the booster itself sliding off.
This could be a fun thing to try and a nice challenge, but the solutions that use extra motors (with delays and ejection charges) are doubtless more reliable and practical.
Another way would be to use a timer and an ejection canister (as in dual deployment systems) to handle the booster's chute ejection.
You could even do a rear ejection from the booster, triggered by a burn string across the front of the booster's motor. That way, the same particles that ignite the sustainer (or the sustainer's motor) would burn the burn string. Thus, the sustainer would ignite, kicking the booster away, and the booster would (at approximately the same moment) kick out its parachute (or streamer) via a spring (or something similar).
Let your thoughts run wild! Have fun! Get really creative!