Resonance is a relationship between vibrational frequencies, amplitude and time which can be incredibly complex and yet is able to be reduced to mathematical equations. Resonance is something that you feel and appreciate regardless of your understanding of the complex relationships underlying it.
Sometimes the resonance of singing bowls played together with mallets or strikers will produce tones that each individual singing bowl does not have. Some individuals note greater sound healing potency with multiple bowls.
I sell a few singing bowl sets that have strong resonance. Big premium quality jambati are the easiest ones to experience resonance with though any pair of singing bowls can have this feature. One of the great pleasures of having a singing bowl collection is to play with your pieces and see which ones really go together.
Finding Good Vibrations
Singing bowls that are close in tone will sometimes set one another off (ring one and silence it and you can hear the other faintly ringing). Even if the singing bowls don’t set each other off playing two or three singing bowls that are close in tone produces a nice effect.
You often get an augmentation of the main tone and a pulsing as the singing bowl motions increase and decrease the amplitude of the sound wave.
Greater Than The Sum of Parts
I do not think it is possible to test multi-bowl resonance on the web. With singing bowls that are close in tone you have a high likelihood of the whole sounding greater than the sum of the parts.
For singing bowls with different tones resonance is more of a mystery. Take it as a gift if it shows up in a group of bowls you have or buy. As noted earlier, antique brass instruments of all kinds have deeper, richer tones.