Actually a couple of advantages, organization and numbers. For two Asha'man, then yes that is exhausting, but Seanchan have huge numbers of Channelers. As well they wouldn’t need to bring their supplies with them thus shortening the time required to keep Gateways open per each “jump”. The Seanchan would likely organize specific times for their Damane to open Gateways at supply depot, allowing for a quick resupply without the need to carry it with them. Plus if the Damane were getting tired they could gateway back to the kennels and exchange the tired Damane for a fresh Damane. Therefore the Seanchan could easily, efficiently and quickly move small bands anywhere they wanted. If you attach a Damane per 1000-2000 troops in the field you could move at blinding speeds (relatively). Attach 100 or so of these bands together forming an army and you could move 100000-200000 in hours without the need to move supplies. To me this sounds very effective.

Basically the Seanchan only have to set up an organizational system for resupply and a rotation for the Damane for Gateways to be a huge advantage for them.