man schafft was man will, auch zu zwei'n, http://www.writeatext.de/text/text/4446
Glück allein oder zu zwei'n
Schön ist das Leben nur zu zwei'n
Just like in English, an apostrophe designates an omitted letter. In this case, it's an E. It sounds rather archaic though, at least where I am from. Normally you'd go with zu zweit these days.
I'd say it means zu zweien. But you have thereafter sein, which wouldn't rhyme, if you put that instead of zwei'n. In case that the metric of the text would be important the apostrophe would be attributed to trying to cut a syllable. But here that doesn't seem to be the case.