Perk to remove the -1 penalty for "spells on"

[Joe]

In the interests of clarity, I'll begin by stating the obvious: when casting a spell, you suffer a -1 penalty for each other spell you currently have "on" (as per p.B238).

Now, I seem to remember that there's a perk written up somewhere that allows you to avoid that penalty, for a single spell - i.e your attempts to cast other[I] spells are not penalized by the fact that you're maintaining this spell.

Am I remembering this correctly? I may well be mistaken, since I've looked through my books and can't seem to find it anywhere. (And I can certainly imagine it generating balance issues, if not handled sensitively)

Thanks in advance!