Why will 2 similar objects moving at the same speed side by side tend to drift closer together?

Gravity?

Correct, all objects with mass have a gravitational field. It may also have something to do with pressure, I am not sure...

I have a hard time visualising what's happening Juicer,

Could you provide a better idea of what you're thinking of? Are these objects moving in a vacuum? What kind of setting is it?

I thing relative air speed between object is more high, when they close, so preasure(bernoulli... ) is law.so they come more closely.