Imagine a "master" Arduino (M) which is connected to 2 other "slave" Arduinos (SA & SB).
Imagine that 4 bits of all three Arduino's are connected together in parallel. The master M always has its bits as input. And by default the slaves have their bits configured as inputs.
Imagine that one of the two slaves (say slave A) has to send 4 bit data to the master. Can I make slave B's bits configured as input then output the data from slave A?
Ignore the problem of having two slaves outputting at the same time, that would never happen for mechanical reasons.