I am thinking of an NPC mage with Flexible Formulaic Magic and Enduring Magic, who can easily cast "Maintain the Demanding Spell" flexed up to Sun duration, while he uses FFM to reduce other spells to Concentration duration for easier casting.
However, if he wants to take manual control of one of the spells he is Maintaining, how easy is it to select the correct spell?
The standard seems to be that you know what you're concentrating on. But I have played in campaigns that required you to better identify effects if you have a bunch of them. In that case InVi just like Sight of the Active Magics should work, though you can drop it from Vision to Touch since you're considered to be touching these spells, and that will drop the level to 25.
You might consider a Sun version of Maintaining the Demanding Spell, as you can flex that down to Diameter or up to Moon.