No, with existing Webhook alert action it is not possible. However you can create custom alert action to send data to multiple webehook URLs.
Check this: https://docs.splunk.com/Documentation/Splunk/8.0.2/AdvancedDev/CustomAlertConvertScripted#Migrated_c...
This example explains about creating custom alert action to send splunk event data to one Slack team room. But you can make some changes in python script to send events to multiple team rooms.
