Question About Concentration

[ATHATH]

Casting another spell that requires Concentration. You lose Concentration on a spell if you cast another spell that requires Concentration. You can’t concentrate on two spells at once.
Do you lose concentration on a spell as soon as you choose the target for a different concentration spell, start casting a different concentration spell, or when the second concentration spell comes into effect?

For example, say I have a Rogue friend. I cast Polymorph on him to turn him into a turtle, then try to cast Beast Bond (or Dominate Beast) on him. What happens?

[PhantomSoul]

According to Sage Advice, the intent is that starting to cast another Concentration spell breaks Concentration on the (formerly) active Concentration spell.

https://www.sageadvice.eu/2017/06/30/if-you-have-a-concentration-spell-and-cast-an-offensive-con-spell-that-failed-due-st-do-you-loose-your-initial-spell/

If you're concentrating on a spell, your concentration on it ends the moment you start casting another concentration spell.