Yes, you can. Some Bombardino recipes do, in fact, use equal quantities of both. However, using a little less brandy really does allow the flavour of the advocaat to shine through whilst still providing the 'heat effect' of the drink to work its magic. I've gone for 2 parts advocaat to 1 part brandy, but you can increase the brandy up to the 50:50 ratio to suit your tastes.