Assuming Wickham was not actually staying with Georgiana and Mrs Younge, which was probably the case, but staying nearby, once Georgiana had confessed to Darcy of the intended elopement, she no doubt would have also told him of Wickham's directions. By writing to Wickham and saying 'go away or else' - or whatever he said to make Wickham leave immediately :) - Darcy prevents the matter from becoming public knowledge, which was very important. It is unlikely they could have met publicly without someone noticing a heated conversation between the two men, or even overhearing their conversation. If Darcy had suggested to Wickham they meet, he would have been unlikely to show up: more so because such an offer would also suggest a 'meeting by appointment' - i.e. where Darcy would be aiming to punish Wickham's conduct via a duel - and I cannot see Wickham as the sort of man who would show up in such a situation !!