How could you change a sentence having two objects into the passive voice in English ?

Easy. Use one of these objects to start with. Hence, you can make2 passive transformations for such a sentence. 

e.g.

I gave him a rose.

>> He was given a rose by me.

Or >> A rose was given to him by me. {Notice inserting a preposition (to or for) before the indirect object}

https://learnenglish.britishcouncil.org/en/english-grammar/verbs/double-object-verbs