Can I write two time signature at the same time?

Also/or: time sig.mscz
Ie Special Characters (Musical Symbols/ Time Signatures) -> add 9 then 8 (size 20).
Add this 9/8 when you had finished to enter your score (to avoid the moves due to the layout)
Alternatively, add an image (Captur Tool)

Same idea as this one I have in my palette?

Press shift T to get the time signature palette then make a decision. Do you want this to represent 6/8 or 9/8, you must decide that. Set the value to that then set the text to 69 / 88. Adjust the beams and click the add button at the bottom and you can then drag it to your score or custom palette.

You will then have to adjust each measure with the other beat count. If you decide that this will represent 6/8, then on every 9/8 measure you will need to adjust the actual duration to 9/8. Right click the measure, select measure properties and change the 6 in actual duration to a 9. Unfortunately you can only do one measure at a time. If you copy measures it will not replace the destination measures duration, it will only paste the rest pattern at the destination.