I know you can do it.
When you get to the screen to add the tags, just drag the size box you want, and have another tab open so you can go to the database for that plant. On the left side of the database entry scroll down and you will see an ID number for the plant, copy and paste it into the box you have created on the photo. When you have both plants labeled you can click on 'Finished adding tags? View the image.'
All done.