Monitoring via headphones whilst main output active

Hi, I am using an Alto ZMX862 with a pair of Alto Trouper speakers. I can monitor the mix using headphones but only when the speakers are switched off; as soon as they are switched on there is no output from the headphones. Is it possible to have output from the main mix via the speakers and also monitor this using headphones at the same time? The Main/2-track button doesn't effect this. Many thanks.
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