"Two male students"
дві is the plural, два is the singular. If you want to get more complex though and add cases, look at this: http://ukrainiangrammar.com/article/2012-06-08/the-numeral-cases.html
Два is used with masculine and neuter nouns, дві is used with feminine nouns.
дві is the plural, два is the singular
Almost. :) You're correctly using the endings: сестра 'sister' is singular, сестри 'sistes' is plural. However, два/дві is not a usual word, so this doesn't work in the same way for it.
'Two' doesn't have singular form, because its meaning would conflict with it: the singular form is used for one thing, and 'two' is used for two things.
The link you've given has the correct declination.