Preheat the smoker to 250°F. If needed, prepare it according to the manufacturer’s instructions.
In a large mixing bowl, whisk the egg and milk. Stir in the breadcrumbs and let them sit for a minute to absorb the liquid.
Add the ground beef, Italian sausage, Parmesan, garlic, Italian seasoning, salt, and pepper. Mix gently with your hands until just combined. Avoid squeezing or overmixing, which can make the meatloaf dense.
Shape the mixture into a compact loaf and place it on a smoker-safe tray or a piece of foil. Leave space around the loaf so smoke can circulate.
Smoke until the center reaches 145°F, then brush the top and sides with the marinara and barbecue sauce mixed together. Continue smoking until the center reaches 160°F.
Remove the meatloaf from the smoker and let it rest for 10 minutes before slicing. This helps the slices hold together and keeps the juices in the loaf.