Keeping the two devices close together will help prevent unintentional interference. If they are close, it's less likely that something else, further away, could interfere with a transmission already in progress.
However, wifi devices will wait for the physical medium to be free - some sort of carrier-sense multiple access. Another device waiting to talk, or already talking, will cause your devices to wait for their turn.
They will do this for almost any signal, even if it's quite weak.
One solution is to first reduce the range or sensitivity of your wifi devices, and then hold them close together.
For anything with an external antenna, it's easy, just add aluminium foil until it can only communicate for a few metres.
For a device with an internal antenna, like a phone or camera, you need to be creative with the foil. Search for images from a teardown to locate the WiFi antennas? Experiment with your hand or a small sheet of foil? Cover most of the camera, except lens and controls?
Once you have two devices whose range (to a normal AP) has been reduced to a few metres, do one last check: test that they can still talk to each other, when held 1 m apart. Then when you put them 10 cm apart, they are sure to have enough signal.