The shared magic gauge is a gauge separate from your individual magic gauges. It is located in the upper center of the screen, and you want to fill it up whenever you can.
How does one fill the gauge? By attacking the same enemy! You’ll know that’s happening when you see a rainbow infinity symbol covering the enemy, and also present inside of the gauge itself. As you attack the same enemy, the gauge will fill blue.
When the gauge is completely full, you and your partner are awarded with bonus Umbran Climax! You’ll automatically go into Umbran Climax state for eight full seconds, and it will not drain your individual magic gauges to use—only the shared magic gauge. Work together with your partner to really be able to put the hurt on your foes!