For example, you throw a coin, shoot it and it hits 2 enemies simultaneously. Seen this thing a lot in many speedruns, but can't manage to do it myself, no matter how much I tried to

Pretty simple actually, once you throw the coin, it should appear a yellow flash on it like 1 second after throwing it, shoot it once the flash appears and it will hit the 2 nearest enemies

Oh damn. Thank you

The same thing happens after the coin has been active for about 3-4 seconds

There are two windows where a coin can create a splitshot. The first is a very brief period after launch where the coin produces a visible flash and makes a twinkling noise. Shooting the coin just as this twinkle appears will create a splitshot. The second opportunity only begins after the coin has been airborne for a while, and is indicated by the coin making a distinct whistling noise, commonly called Singing by the community, as it falls. Once achieved, this window will last indefinitely until the coin expires.

late splits are pretty easy to do but take a while, the best advice i could give for early split timings are to go into sandbox with all the cheats you need and just try and get a feel for it until it's dented into your head, you could also try relying on a cue to make it easier