When reading about Miss Bingley's 'kind' warning to Lizzy, I was struck by the fact that Bingley had never seen Wickham in person. Therefore, I feel that Darcy and Bingley were acquainted after Wickham stopped being part of Pemberley, ie, only for the past few years. Moreover, Bingley seems to have been of age only for 2 years. Perhaps, he got introduced to Darcy through some common 'connection', and thus their friendship began...