How to correctly pronounce Xenial Xerus?

They are both dictionary words

xenial xerus (sound clips included here)

zee-nee-ul zeer-us

or in IPA

/ˈzi:nɪəlˈzɪərəs/

It follows closely the Xubuntu pronunciation that you've referenced.

Zee-ni-al Zee-rus

Would be the best way to describe it.

The same as it sounds at Google Translate when the English language selected. Click on the speaker icon at the bottom of the input text box to listen to it pronounced. It sounds like zee-nee-al zee-rus with the accent on the first syllable in both words.