The incidence of ANCA-positive disease is in the ballpark of 1:10,000. The incidence of anti-GBM disease is even more rare, at about 1:1,000,000. One would expect if these 2 disease-causing antibodies were entirely independent of one another, the incidence of a "double positive" (e.g., both ANCA & anti-GBM-positive) would be predicted to be (1:10,000 x 1:1,000,000 =) 1 in 10,000,000,000, or one in 10 billion.
Double positive disease is much more common than this, however. One study by Levy et al in a 2006 Kidney International paper showed that 5% of all ANCA-positive serum samples were also positive for anti-GBM antibodies, and about 1/3 of all anti-GBM positive samples had detectable ANCA. Most of the time (82%) the double-positive patients had an anti-MPO (myeloperoxidase) specific antibody. Perhaps not surprisingly, the double antibody-positive patients have a worse outcome than single-positive patients.
One theory to explain the existence of double-positivity is that the disease starts off as an ANCA-mediated process and damages the glomerular basement membrane, thereby exposing antigens to which an anti-GBM can form. Treatment in the acute setting is generally aggressive (e.g., treat them as if they have anti-GBM disease) and chronically is often managed as an ANCA disease.