How common is it for a toddler to have both a urachal cyst & meckel's diverticulum?

Not very common.: Urachal cysts are quite rare; meckel's diverticulum occurs in 0.2-4% (but only 5% of people with it have problems from it). So to have both is uncommon!

Sometimes: Wow. What a question. My research is that both originate embryologically from the omphalomesanteric duct. One source recommends a work up including a scan to look for a meckels if a urachal cyst is present since the presence of the later suggests more omphalomesanteric abnormalities...But the frequency of both i didn't find. However most meckel's are asymptomatic and not likely to cause problems.