If i keep a candle under a small chimney and a smoldering incense stick under another one what will happen?

The candle will heat the air around the flame. That warm air will expand so that its density becomes smaller. The low density volume of air will experience an upthrust from the cool ( more dense ) surrounding air and so will rise up the chimney. As it moves away from the candle, the surrounding cool air will move in to take the warm air's place. IT will heat and rise and a flow of hot air up the chimney and cool air towards the candle will establish itself. This is called a convection current and you will be able to see the flow because the incense smog will follow the flow. ( the incense is only there to show you the flow of the air ). The exiting hot air from the candles chimney will be replaced by cool air coming in at the incense sticks chimney.