double worked plum

double worked plum. on the rootstock is a short length of Prunus americana, and then on top Yakima plum, a cold hardy european plum. will it survive -40? we'll find out, it's in a semi sheltered location, five ft off the ground, and on a super cold hardy rootstock (chokecherry), all of these factors might improve winter survival. just in case I'll cut scions in november.