Since in you're example you're multiplying the fraction by 100, i'm assuming the subdept
value would only have two digits. If it's the case, perhaps you could try the following code:
public IEnumerable<InventoryItem> GetDepartmentRange(double deptBegin, double deptEnd, string dbContext)
{
LoadInventoryItems(dbContext);
return inventoryItems
.Where(d => ((d.dept * 100 + d.subdept) >= deptBegin * 100) &&
((d.dept * 100 + d.subdept) <= deptEnd * 100))
.OrderBy(o => o.dept)
.ThenBy(s => s.subdept);
}