Read both clefs at once: name the bass note for your left hand and the treble note for your right, then lock them in before time runs out.
A grand staff shows one note on the treble staff and one on the bass staff simultaneously. Pick the letter for each hand and lock both in before the timer empties. Getting both right builds your streak and a speed bonus across twelve rounds.
Two-Hand Tally builds genuine grand-staff fluency by asking for treble and bass notes at the same moment.
Reading a treble and a bass note simultaneously off the grand staff is what a keyboard player does continuously.
Requiring both before the timer, rather than one at a time, is the original demand.
Simultaneity is the actual skill and it is not the sum of two easier ones. The two staves use different clefs, so the same-looking note means different pitches in each hand, and the reader must hold both mappings at once rather than switching between them.
Two-Hand Tally trains genuine grand-staff fluency by asking for treble and bass at the same moment. It is an original, free browser game built in-house for PlayPendium.