At the moment one can not set the maximum screen time limits no matter which device the child uses.
As mentioned on the official Google help site:
Daily limits apply to each Android device or Chromebook your child uses. For example, if you set a daily limit of 2 hours, your child would get 2 hours of time on each device.
As an alternative
If and when the limit is reached on a specific device you could give your child bonus time without changing their daily limit or bedtime schedule.
- Open the Family Link app Family Link.
- Select your child.
- On the card for one of your child's Android or Chromebook devices, the Bonus time Bonus time chip will appear when your child’s device is
locking soon or if your child’s device has already locked.
- Tap Bonus time Bonus time.
- Follow the instructions on the screen to give your child bonus time for the day.