If you have a 3 burner Spirit II and a half size griddle either side should be OK for the fit. Check the grease outlet to make sure you are OK with the discharge location
If you have a 2 burner Spirit II the cooking surface is not a wide as the 3 burner. The distance front to back is the same. So a half size griddle for the 3 burner would take up 60% of the cooking surface on a 2 burner maybe leaving you without a cooking grid next to the griddle
