Yes, if the devices use the same Google Play account you can use it without paying again.
To use Pattern Keeper on multiple devices please make sure that all your devices use the same Google Play account. If the app tells you that it is in trial mode and you already paid for the app – then click the buy option in the top right corner of the screen and in the following dialog choose the “Restore purchase” option.
Please note that Pattern Keeper doesn’t have automatic sync. To transfer your progress you have to export it from one device and then import it on the other.