There is no Biblical basis for two separate killings of two different lives (of the lamb and of Jesus) being one sacrifice.
Do you have any evidence from the early Church Fathers of this non-Biblical notion?
After all those well thought out postings of the Holy Week timeline, I can hardly believe that you say such a thing. But, ....
Ok. Let's go back to the Old Testament.
On Mt. Moriah, which would later be known as Golgotha, a father sacrificed his son in accordance with God's will.
And it came to pass after these things, that God did tempt Abraham, and said unto him, Abraham: and he said, Behold, here I am. And he said, Take now thy son, thine only son Isaac, whom thou lovest, and get thee into the land of Moriah; and offer him there for a burnt offering upon one of the mountains which I will tell thee of. And Abraham rose up early in the morning, and saddled his ass, and took two of his young men with him, and Isaac his son, and clave the wood for the burnt offering, and rose up, and went unto the place of which God had told him. ...
The son carried the wood for his execution, up the hill.
6 And Abraham took the wood of the burnt offering, and laid it upon Isaac his son; ....
The son asked a question which was given a prophetic answer:
7 And Isaac spake unto Abraham his father, and said, My father: and he said, Here am I, my son. And he said, Behold the fire and the wood: but where is the lamb for a burnt offering? 8 And Abraham said, My son, God will provide himself a lamb for a burnt offering: so they went both of them together.
Just when Fr. Abraham was about to give up his son's life:
11 And the angel of the Lord called unto him out of heaven, and said, Abraham, Abraham: and he said, Here am I.
12 And he said, Lay not thine hand upon the lad, neither do thou any thing unto him: for now I know that thou fearest God, seeing thou hast not withheld thy son, thine only son from me.
And the lamb of God was provided for the sacrifice:
13 And Abraham lifted up his eyes, and looked, and behold behind him a ram caught in a thicket by his horns: and Abraham went and took the ram, and offered him up for a burnt offering in the stead of his son.
This was the first foreshadowing of the Lamb of God's sacrifice on Golgotha.
John 1:29 The next day John seeth Jesus coming unto him, and saith, Behold the Lamb of God, which taketh away the sin of the world.
Fast forward to the Passover where God said:
Exodus 12:3 Speak ye unto all the congregation of Israel, saying, In the tenth day of this month they shall take to them every man a lamb, according to the house of their fathers, a lamb for an house:
4 And if the household be too little for the lamb, let him and his neighbour next unto his house take it according to the number of the souls; every man according to his eating shall make your count for the lamb.
5 Your lamb shall be without blemish, a male of the first year: ye shall take it out from the sheep, or from the goats:....43 And the Lord said unto Moses and Aaron, This is the ordinance of the passover: There shall no stranger eat thereof:
44 But every man's servant that is bought for money, when thou hast circumcised him, then shall he eat thereof.
45 A foreigner and an hired servant shall not eat thereof.
46 In one house shall it be eaten; thou shalt not carry forth ought of the flesh abroad out of the house; neither shall ye break a bone thereof.
Scripture ties this back to:
John 19:33 But when they came to Jesus, and saw that he was dead already, they brake not his legs:
34 But one of the soldiers with a spear pierced his side, and forthwith came there out blood and water.
35 And he that saw it bare record, and his record is true: and he knoweth that he saith true, that ye might believe.36 For these things were done, that the scripture should be fulfilled, A bone of him shall not be broken.
The confirmation that the Apostles understood that Jesus is the Lamb of God who was sacrificed for our Christian Passover are in these words:
1 Corinthians 5:7 Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our passover is sacrificed for us:
That also links the sacrifice of Christ, the Lamb of God, to the unleavened bread of the Eucharist. As well as these words:
John 6:51 I am the living bread which came down from heaven: if any man eat of this bread, he shall live for ever: and the bread that I will give is my flesh, which I will give for the life of the world.
But that's another lesson. Suffice to say, Jesus is the Lamb of God who offered His life for the salvation of the world. Since Jesus is God, only His life would suffice:
Hebrews 10:4 For it is not possible that the blood of bulls and of goats should take away sins.
Does that help?