I must admit that your question completely took me aback. Moreover, when I consulted Longman Dictionary of Contemporary Usage and Practical English Usage I failed to find anything that would explain the appearing and disappearing coma before "too" and "either".
What added confusion is that in all the examples provided by Longman there are plenty of "too" and "either" with and without commas. I had to ask my friend Alexander to lend me a hand with it.
Luckily, Alexander managed to find the answer in the Chicago Manual of Style. You can check it yourself if you wish at http://www.chicagomanualofstyle.org/cmosfaq.Commas.html
But here is the answer to your question from the Chicago Manual of Style.
Question. Please help clarify a debate over what I see as a groundless but persistent carryover from high-school English classes: the comma-before-too “rule.” The rule goes something like this: When “too” is used in the sense of “also,” use a comma before and after “too” in the middle of a sentence and a comma before “too” at the end of a sentence. I am editing a work of fiction in which the author has rigidly applied the rule. I have just as rigidly deleted the commas. My managing editor believes that a comma is needed when “too” refers to an item in a list and has the sense of “in addition” (e.g., “I like apples and bananas, too.”), but she would omit the comma when “too” refers to the subject of the sentence (e.g., “Oh, you like apples and bananas? I like apples and bananas too.”). My managing editor’s rule helps make a useful distinction, but I am still wondering whether the comma is ever grammatically justified.
Answer. A comma can do some work in making the meaning of a sentence clear, but to claim two different meanings for I like apples and bananas too with and without a comma before too puts too much pressure on the comma. Out of context, neither version would be perfectly clear. To make the different meanings more apparent, short of additional context, you’d have to be more explicit:
I too like apples and bananas.
I like not only apples but bananas too.
Use commas with too only when you want to emphasize an abrupt change of thought:
He didn’t know at first what hit him, but then, too, he hadn’t ever walked in a field strewn with garden rakes.
In most other cases, commas with this short adverb are unnecessary (an exception being sentences that begin with too—in the sense of also—a construction some writers would avoid as being too awkward).
I hope this helps. Let me know if you need any help.