I don't recommand serving with two balls in the hand during in a match or in practice. The ball could slip out during play or practice (some players do dump it on the court, don't recommend that either, the player could trip over it), some carry it in their hand while they are in play, in either case, the spare ball could slip out and get under a player's foot or their partner's foot during the heat of an exchange and cause them an injury.
So I consider it valuable only to hold two balls in the tossing hand as a training tool just to get used to holding the ball on the fingers for the serve. But when it comes to practicing your serve or serving in a match, you use only one ball to toss with not two.. Because two or three ball in the hand tossing is a dangerous way to serve, for during the serve the ball could slip out and roll under the front foot as the toe lifts up to serve and come back down on a stray loose ball and cause a nasty foot injury or if it rolls towards your partner cause them to trip over it and injure themselves.
If club players and veterans are practicing serving with two balls in the hand throwing one up while holding onto the other, then they should discontinue this practice and go back to the proper correct modern day serve, tossing only with one ball in the hand and keep the other ball safe in their pocket so they don't waste time fumbling about and not increase the risk of injury.
or leave a stray loose ball on the court for someone to trip over..