The words for the number two are remarkably similar throughout the indo-european languages, the basic pattern being a frontal plosive ([t] or [d]) in the beginning, followed by a vowel that is usually [o] or [u].
In the Germanic and Slavic languages, the [o]/[u] sound has been reduced to a [w] or [v] sound after the addition of a following vowel. The can presumably be counted as the [u] sounds they originally were.

Due to the language shift that created Modern High German (the main dialect of Modern German), this language has [ts] instead of [t], which should also be ignored to decrease the word's complexity.

The most common pattern should be retained, leaving the following options:

tu, which is already used for the 2nd person singular pronoun you
du, which is the one closest to the variants used in the romance languages and to the [v] and [w] sounds in Slavic and Germanic
to, this sounds nothing like any variant of the name of the digit 2 in any european language
do, same issue as with to