ITN - EJD: One or two tuition fees for an ESR?

If in an EJD (European Joint Doctorate, Innovative Training Networks) the ESRs have to pay tuition fees, do they have to pay 2 tuition fees (1+1 where the ESR is developing the PhD) or just 1 tuition where s/he is recruited? In cases where the ESRs are enrolled in institutions which have a high tuition fee this might be a very important question. A best practice examples could be to charge the doctoral candidate the tuition fees by his/her 'home' institution, but the same candidate is not charged full fees by the second institution awarding the joint doctoral degree. Instead they pay a processing/administration fee, e.g. 100 Euro.