No later than 3 years after provisional admission to your doctorate, your 1st co-examiner must be appointed and approved by the Doctoral Committee.
In this context, your immatriculation date as a doctoral student is decisive:
prior to 1 July 2016: previous "Additional Provisions D-BAUG", (PDF, 89 KB) section 4.2 Co-Examiners, version 01.11.2013.
after 1 July 2016: new "Additional Provisions D-BAUG", (PDF, 107 KB) section 4. Co-Examiners, from 01.07.2016
Official Requests for Co-Examiners are to be e-mailed by your supervisor to Karin Schneider at the D-BAUG doctoral administration: , providing her with all the necessary contact details, especially of external co-examiners (see point 4.4.)
The requests for co-examiners are dealt with in the 4 annual meetings of the Doctoral Committee. (See dates in grey section on the far right of this website.)
NB: The requirement external University Professor does not have to be met until the "final assessment of the examination committee", but in good time before sitting the doctoral examination. Therefore, your first co-examiner could easily be an internal specialist at ETH.
If an external University Professor has still not been officially appointed by the planned year of your doctoral examination, the meeting dates of the Doctoral Committee are to be taken into account in order to get the person officially nominated.