If you're breaking things up visually using whitespace, that would scream to me that you should be using a more obvious method:
>>> class AcceleratorTable(object):
... @property
... def file(self):
... return [('a', 'b'), ('c', 'd')]
... @property
... def project(self):
... return [('e', 'f'), ('g', 'h')]
...
>>> t = AcceleratorTable()
>>> t.file + t.project
[('a', 'b'), ('c', 'd'), ('e', 'f'), ('g', 'h')]
Now you're not asking questions about whitespace and you've increased readability. While yes, there's slightly more overhead due to function lookups, this isn't something that needs a ridiculous level of optimization. In my mind, this would be a much more Pythonic solution than what you currently have.
To answer your actual question, whitespace is fine to break things up visually, but I'd recommend that the number of lines that you use should not exceed that of the number of lines between other entities in your code (such as number of lines between functions within a class, or module-level classes or functions). As Python's readability is so heavily based on whitespace, using more whitespace in a particular method to break up internal code might initially throw off a reader looking to grok it at a glance.