Blues and twos?

[whiplash]

I know what the blues are,but what are the two's?

[chrismatt.]

It refers to the siren on a police vehicle, which typically has a two tone sound effect

[Little Londoner]

I know what the blues are,but what are the two's?

It was the two tone horns used on emergency vehicles.

[said]

I know what the blues are,but what are the two's?

Could it mean two Police Officers in a Police Car? You know, like the Blues Brothers?

[jonnybgood]

Blues and twos is a colloquial British phrase referring to the blue flashing lights and two-tone siren of a police car or other emergency vehicle which is responding to some incident (although most sirens now have a range of tones like Wail, Yelp and Phaser).