MIDI Thru 1in -> 4out splitter PCB or a Kit from Deftaudio on Tindie
MIDI Solutions Quadra Thru V2 – Thomann United States
Amazon.com: CME WIDI Thru6 BT - 2-in-6-out MIDI Thru/Split with 5-pin DIN and Bluetooth MIDI - future-proof technology, high-speed transfer of all MIDI data, extreme accuracy for all MIDI devices : Musical
The MIDI Standard: Introduction to MIDI and Computer Music: Center for Electronic and Computer Music: Jacobs School of Music