The Harbinger and the Fountain
You can make any number of powers you want but you have to spend Fate points temporarily for access to that power during a scene. Mundane spells cost no Fate Points, Extraordinary cost 1 Fate Point each, Superhuman cost 2 Fate Points each, Ascendant cost 4 Fate Points each, etc. These Fate points are refunded to you at the end of the scene to be used again in the next one. You can make magical effects up to the Tier your skill is at (Superhuman, I believe). Once you spend the Fate point for the spell you can use it over and over and over again. If you want something different, you will need to spend more Fate points.
In addition, it seems like having only one spell be able to be effective at a time is kind of a bummer so I am borrowing something from the Dresden Files RPG. When you roll your magic roll you generate a number of successes. These successes can be used to purchase an extended duration at the cost of being less effective now. For example, you cast your Chains of the Faithful spell at a power level of Superhuman (pay 2 Fate points, please). You roll your dice and come up with 7 successes. For a spell that you need to maintain every round, the bad guy needs to beat a target roll of 7.
But lets say you want to do something else with your magic while you have this fellow held. You can bleed off successes to make your spell last longer. In the book there is a Time Ladder that goes up and down in shifts. Using a default duration of your spell as a Full action you can bump the time the spell works for every success you spend to make it last longer. The time table is:
Free Action/No time
An Action/An Instant
*Full Action/A Few Moments (This is the default of your spells)
Two Rounds/Half a Minute
Three Rounds/A Minute
A Conflict/A Few Minutes
A Scene/15 Minutes
With the previous example, you roll 7 successes and decide you want this spell to last longer so you can focus on other things. You spend 3 of your successes moving it down the table three shifts to “A Conflict/A Few Minutes”. Now you can wander around and do other things too without having to keep the spell going. Because you spent 3 successes on duration, however, he only has to roll against a target number of 4 (7 successes – 3 successes spent on duration).
Addition: Because Profane magic is for short, rapid effects I think we should limit the amount of time a spell lasts to a single conflict only.