I'm very surprised to get only one result for "two same ideas" on Google Books, especially because the result appears in a grammar book (Everyday Grammar by Irene Chong):
Molly and Jimmy are good friends, (and – to join two same ideas)
I know that "same" there can be replaced with "similar" and I verified that "two similar ideas" is rather common both on Google Books and on the most important American and British online newspapers.
Also, in the same way, I verified that "the same ideas" is grammatical.
So, I concluded that there is something wrong with the word "same" in "two same ideas", but I cannot understand what.
Question is: Why are "the same ideas" and "two similar ideas" correct while "two same ideas" is not—or, at least, is so uncommon?
Best Answer
If two ideas, for example, are similar, the implication is there really are two ideas - that have many points in common (though sometimes the points in common may be very context-specific).
If two ideas are the same, the implication is there's actually only one idea (albeit perhaps presented in two very different forms).
This distinction is commonly understood. Thus, for example...
...are both valid. In (1), the implication is one single point is made by both essays (either or both essays may make additional points which aren't in the other). In (2), the implication is more than one point is made in both essays.
Thus, there's nothing wrong with saying "We both have the same ideas" - it just means we have more than one idea in common. But no-one ever says ?"We both have two same ideas". If the meaning is there's one idea [which we've each thought of independently], it's "We both have the same idea".
If the intended meaning is we've each independently thought of "Idea A" and "Idea B", English doesn't really have a succinct unambiguous way of expressing the concept using the word same. You'd probably rephrase to something like "We have two ideas in common", or "We share two ideas".
That's at the semantic level. At the grammatical level, all I can say is standard usage is at least consistent - we never say ?"This is a same idea", or ?"Here are three same coins". If several things are the same, there's really only one of "it" (within the current "frame of reference"). That's why it's singular, and why it can't take the indefinite article (to speak of "an X" implies the existence of multiple X's).