It is called "undue influence", which most drafters of wills, trust and fiduciary relationships try to avoid. Presumably, Darcy and Georgiana are the only beneficiaries of their father's will. By placing Darcy in the position as the sole arbiter of Georgiana's share of the fortune, he could exert undue influence over her, investing her share inappropriately or otherwise dissipating the assets owed to her and which would come to her upon marriage or when she reached the age of majority (and in case of her death, which would come directly to Darcy -- we all can invision where that may lead with an inscrupulous person). By placing Colonel Fitzwilliam as a co-guardian, any appearance of impropriety or undue influence by Darcy over Georgiana and her share of the fortune is substantially diminished. Thus each guardian has the right to call for an accounting by the other of the expenditure of funds, investments, etc., hopefully thereby assuring retention of Georgiana's fortune for her use.

Of course, our Mr. Darcy is such an upstanding and fine young man he would n'er indulge in thoughts of such impropriety, much less act! But a lawyer's duty is to provide a certain measure of surety....