First of all, the sacrifice of the cross and the sacrifice of the Eucharist are one single sacrifice. The reason for having separate consecrations over the bread and the wine is this. Since Jesus' death resulted from His blood being separated from His body, the two separate consecrations are a sign of Christ's death, and a re-presentation of His act of sacrifice in an unbloody manner.

Does this mean that when I receive Holy Communion, I am receiving a dead Jesus? The answer is no! When the priest mingles a piece of the Host into the chalice filled with Christ's blood at the Agnus Dei, he symbolically shows the resurrection of Christ, since the Body and Blood are mingled as in human life. This means when we receive the Blessed Sacrament at Communion time in a state of grace, it is the living Jesus whom we consume and become a part of.