This is impossible. An isogeny between prime-order curves requires a trivial kernel or a kernel of size equal to the prime order, which would collapse the curve into a trivial group. E1 and E2 have distinct J-invariants, so no non-trivial isogeny can exist. It is mathematically impossible
Do you mean no such isogeny can exist?
That's incorrect.
No such isomorphism (which can be thought of as isogeny of 1 degree) can exist. but isogeny of other prime degrees can exist having different j-invariants.