I can think of a couple of possible reasons:

First, two heads are better than one, particularly if they happen to be fairly young heads themselves. Each will be a check on the other's judgment, and each will have the right and duty to keep an eye on the disposition of Georgianna's money. Fiduciaries have been known to abuse their trust, and having two is a sensible precaution.

Second, it provides for the contingency of one dying. To be sure, Darcy and Fitzwilliam were healthy young men, but healthy young men in those days fought duels, upset their carriages, got run down by the Clapham omnibus, and caught pneumonia. If there were a single guardian, he died, and Darcy Senior's will didn't provide for a replacement, then the Court of Chancery would have the power to appoint the new guardian. This would be very undesirable; her person and property (including the right to approve her marriage during her minority) would pass under the control of the Chancellor and a guardian appointed by him, who might be a total stranger to the family. The new guardian would receive such fees from her estate as the Chancellor approved. Moreover, Chancery's procedures were excruciatingly slow and very expensive.

Any prudent, competent lawyer would try to avoid a vacant guardianship, and joint guardians were one way to do it. If the will or settlement were properly drafted, the survivor of the two guardians might be authorized to appoint a replacement for the one who had died, thereby preserving the security against lapse.

As to why Fitzwilliam instead of the Old Earl or the Young Earl, I don't have a theory.