Who sits upon the throne? -> The king.
Who is the king? -> Jesus.
What is the lamb? -> It is a role that the king played for the purpose of salvation.
Only one can sit upon the throne.
So the king and the lamb are the same person - Christ Jesus.
The phrase `unto him that sits upon the throne and upon the lamb` tells us that the HolySpirit is the one who deserves `blessing and honour and glory and power` for His work in the lamb, Christ Jesus. Note the use of the words `unto` and `upon`. We give worship unto the HolySpirit through worshipping Christ Jesus upon whom the HolySpirit has given glory.
Many people attempt to find the Lord and worship Him. But the HolySpirit (God) cannot be found except through the person of Jesus who is the StraitGate and the NarrowWay.
Every creature that has ever lived will one day praise the majesty and His lamb.
Jesus is no longer a lamb. He is MightyGod. When reference is made of Him on the throne and Him as the lamb, we need to understand that the one on the throne is the lamb. But His role as lamb is finished.