Memento Yearbook currently allows you to design on a single album side. Should you need to design a double-spread with images & text across the gutter, position content off the edges of facing pages.
First design the left side of your page, positioning frames extended over the gutter. You won't be able to see the part that is hanging to the other side of the page. Once the frames are in position, select any content that needs to spill over to the right page, and copy it (Ctrl-C on Windows, Cmd-C on Mac).
Click Edit on the right side page, then paste the copied content to that page (Ctrl-V on Windows, Cmd-V on Mac). Position the frames so that they are aligned to the other side of the page.
In the following example, these two pages have a duplicate photo and text frame positioned off the gutter edge of adjacent pages.
The two copies are independent of each other... which you can see when the content is repositioned.
The advantage to working in this manner is that you can adjust the frames so meaningful content is kept within the spine's safe area.
If you work on a true double spread with content spilling over the gutter, you may find that the printed book will "eat" letters from your title or squish a person's face in the gutter (even making them into a cyclops!). This is caused by the book's binding consuming part of the page in the spine when the book is bound.
For example, a double spread with an image right in the gutter might print to look like this, or worse. Notice the break in the line of the wall and the cut-off shoulder because the photo is stuck in the binding itself.
By using two copies of the same frames manually positioned on both pages, you can reposition the content slightly to avoid the spine "squish" in your finished book, duplicating content across the gutter slightly to avoid this end result. Use the safe (green) guideline to align photo and text so it is clear of the area that might be consumed in the spine. Slightly offsetting images will ensure that the spine doesn't interfere with viewing the entire photo.