Whole Rest 2nd Voice to center

Hi Everyone,
I found a lot of things about 2nd voice unneeded rests but to solve my problem I am completetly in the dark!
On an orchestral score it's very often to have a flute with 2 voices on the same staff. While the 1st voice plays the 2nd one is muted and vice et versa. That's why you need to have the whole rest written.
My question is how can I center those whole rests of the 2nd voice as it's possible in 1st voice ?
(see attached score)
Many thanks for your help,
P.F.