One agent is assigned to one queue ; so he will execute one task at the time, sequentially, according to their rank in the queue. But you can create as many queues as you want, and assign an agent for each one. You simply fire each agent from a terminal, with a command like :
clearml-agent daemon --queue my_queue_i
if you have more than one gpu, you can also choose for each agent which gpu(s) to allocate