Almost always yes- it of course depends on the people, but you hear each person’s unique voice. Unless they are exactly the same frequency (in which case they would either cancel out if opposite phase or double if in phase), they will likely constructively and destructively interfere with each other. The outcome will almost certainly be louder than not.
As for dB scale, every doubling of sound pressure is 3dB. Some frequencies can excite each other and grow, while some can cancel.
[Here’s another good explanation ](https://physics.stackexchange.com/questions/214485/why-dont-choir-voices-destructively-interfere-so-that-we-cant-hear-them)
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