1. Open two images you want to combine (Tulip and rose in my work).
2. Make a new document (For example 700x700px) and copy both images here (One image per one layer).
3. Select upper layer and click on "Add Layer Mask" button, located under layer list.
4. Now you'll see something like this:
5. Click on blank thumbnail of mask layer.
6. Select Gradient Tool.
7. Leave default - Black and White gradient.
8. Draw line like that:
9. It's done. You can paint on the mask layer to correct masking.