Can I use a single air gap for a reverse osmosis system and a dishwasher?
You have two options which are equally acceptable. 

Option one is a dual inlet air gap. It provides two inlets - one for dishwasher, and another for a water purification system. Installation is straight forward - you remove the existing air gap and put the new one in its place. You can find such a device here.

Your other option is to purchase a faucet for your water filter that includes a built-in air gap, such as this. Note that if you go this route, you will have two drain lines that both need to connect to the garbage disposal. You can accomplish this by using a wye fitting to join the two drain lines into one line, which you would then connect to the disposal. A barbed wye fitting should be available at your local home improvement store for a minimal cost. Don't forget the hose clamps.

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