The Tandems event is a collaborative event where players' efforts earn them different rewards. You can even feed your 4 pets at the same time to hope to reach the grand prize! 
Each pet must reach the fifth stage of evolution, from box (level 1) to full maturity (level 5).  When each level is reached, you and your tandems can earn rewards!
 
Here are a few details:
- The pets remain hidden in their box until you have completed level 1.
 - Each time a pet reaches a higher stage, you and your tandem receive the associated rewards.
 - If your tandems feed your shared pet while you're away, a dot will appear on the Tandems' icon.
 - When your tandems evolve your shared pet, you'll see the pet's latest evolution, and you can collect the associated reward.
 
For more details about our various features, you can browse our FAQs and, of course, if you would like more information on this subject, we invite you to contact our Customer Service by clicking on the Settings tab (with the 3 lines) at the top right of your home screen, then on Help. To open a conversation, you can either click on the bubble 💬 in the top right corner or click on ‘Chat with us’ at the bottom of the page.
We hope you have fun playing!