Users are prevented from adding additional locations to scenarios in a multi-test when attempting to configure the scenario from the BlazeMeter Performance test Configuration tab:
The Total User option was disabled in the Load Configuration section of the scenario in the Performance multi-test.
Component : BLAZEMETER PERFORMANCE TESTING
As specified in the Disable Overrides section of the Load Configuration article, when the Total Users option is disabled, then BlazeMeter will use the configured number of users from the JMX script configured for the scenario.
To add additional locations, the Total Users option must be enabled and the total number of users to use must be configured in each scenario.