As soon as you complete his Making Scents questline, Magnus assigns you this new two-part quest. It is likely that you'll be working Part 1 of this quest around the same time you receive Part 3 of Bluebelle's Asking the Oracle questline. Thus, you may be able to time it such that one trip to the Oracle (including both Magnus and Bluebelle) can knock out quest requirements for both questlines.
Part 1 of 2
Part 2 of 2