If you don't have the grounds of the two arduinos connected, there is no circuit. You need one wire from the signal pin on A to the signal pin on B, and one wire from the ground on A to the ground on B.
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If you don't have the grounds of the two arduinos connected, there is no circuit. You need one wire from the signal pin on A to the signal pin on B, and one wire from the ground on A to the ground on B.
-br