This is only one option for high-low split games; the other is known as "cards speak", in which players simply reveal their hands at showdown and award the pot to the highest and lowest hands shown (possibly subject to qualifications).
Cards speak is used commonly in casinos because it is the much simpler method.
Also, if all the other remaining players declare one way, the last player to declare can then call the other way and take half the pot regardless of the actual rank of his hand.
Simultaneous declarations are commonly done by the "chips in hand" method.
Some games then have another round of betting after the declaration, called "bet/declare/bet", which clearly gives an advantage if there is just one person going a certain way.