Double working refers to making grafts in two locations on the plant. This involves placing an interstock between the scion and the rootstock.
This may be done in certain fruit tree grafts where the scion and rootstock are incompatible, but the interstock is compatible with both the scion and rootstock.
Double working is also the way rose standards are made.
For rose standards, the interstem is budded on the rootstock and allowed to grow for one year.
Then several scion buds (arrows) are budded onto the interstem.