I have to write a page from Massenet with two different Time measure for Voice and Piano ( C and 12/8). How to fix it so that the bar lines coincide ?
Here is how I would do it - if the patterns are as regular. I suspect that's actually what Massenet thought: Triplets! - but why write all those triplets markers? - just put 12/8 in front of it.
So: Write the 12/8 staff as triplets on a C (hidden time signature!) staff, but suppress all the triplet symbols with a "Marker" - created on the keyboad with @ - where "Triplet" is selected; and Visibility is "Never". See attached example.
There might be places where the notation does not really fit - then, you have to fill up with invisible eighth breaks or the like.
(And this will not work if there are triplets on the 12/8 staff - which would be weird, but possible: In that case, you need to change the time signature to 12/8 at those places [or use it throughout]; and write the C staff with "lengthened" notes; the easiest way is adding a dot to each note/chord; and suppress it with "Extra Dot Spacing" set to -1. But this will get you into new troubles if you have dotted notes on the C staff ... which would then require those invisible breaks ... which will, then, make playback "wrong" [which would need separate tempo changes at the C-to-12/8-and-back changes anyway] ... etc.etc.)
Try this tip: Triplets v. Duples, 3/4 v. 9/8, Hiding "3" in triplets (https://forum.noteworthycomposer.com/?topic=6485.0)
Also, a tool to extend the duration of all notes by 1/2 OR add a hidden rest: Triplets.htm (https://wjporter.com/nwc/Triplets.htm)