Copy and paste individual instruments into voice 1 and 2 respectively

A new question. I've been reading the handbook and the forum, but I cannot get it to work.

In a score with many instruments, I would like to save some vertical space in the score by combining the two flutes into on stave with two voices. Somehow I cannot get it to work because it always overwrites voice 1 irrespective what I do . I don't want to implode because I still want to create the individual instruments parts later.

There are quite some postings about this is the forum, but none of them seems to work. , or at least not for me or I'm doing something wrong.

In the attached score, I've copied Flute I and II into individual staves, in the third stave I would like to have Flute I as voice 1 and Flute II as voice 2. NB, I managed to paste the flute i into the third combined stave,.