To run a test involving several servers, I would like to spread the virtual user activity over these machines. For example in a 25 / 25 / 50% split over three machines.
The product documentation does not mention whether this is possible.
In the "Agent Setting" screen of the "Agent Configuration", there is an option to set a maximum number of virtual users or "Max. Virtual Users" for that agent. Set this max value on each agent machine so that the combined values add up to the total number of virtual users you wish to test for, or are licensed for.
Following the example; if you want a 25 / 25 / 50% split on a 500 virtual user test, install a total of 3 agents with max virtual user values set to 125, 125, and 250.
Please note: if additional virtual users are required, please work with Sales/Licensing to upgrade Virtual User Pack