Seconds and Seconds Pro are the same app with different payment methods. Once paid for both apps are exactly the same. However, they are different products and as such they are not interchangeable.
Seconds Interval Timer
Seconds is free to download and test out all the features before committing to purchasing. You may use this version without paying for as long as you like.
Timers created within Seconds may only be used once before upgrading. After using the timer the first time it will become locked and require upgrading before it can be used again. There is no limit to the number of timers you create.
Due to Family Sharing policies the upgrade within Seconds is not available to family members.
Seconds Pro is a paid download. It has no restrictions. It is available with Family Sharing.