is equivalence principle for flat earth or round earth?

According to Einstein, gravity is not a force but earth is accelerated frame, therefore, is it possible for the earth to accelerate in two opposite direction simultaneously, for example, falling of antipodal apples from the same elevation at the same time.

No.

Einstein related gravity to acceleration. It does not mean gravity doesn’t exist.

No means both (The Newtonian and Einstien) mathematical models of gravity are WRONG - Right
And the subject of gravity (why earth attracts) is still MOOTED - Right

Newton was wrong. Einstein’s is incomplete.

No means both (The Newtonian and Einstien) mathematical models of gravity are WRONG - Right
And the subject of gravity (why earth attracts) is still MOOTED - Right

No, it doesn't and Einstein had the greatest respect for Newton's Laws of Motion and Universal Gravitation.
Einstein never set out to disprove Newton. Einstein's intention was to extend Newton's laws to include the finite velocity of light and velocities becoming significant compared to the finite velocity of light.

And whatever anybody tries to tells you Newton's Laws are perfectly adequate for any ordinary calculations unless extreme accuracy or extreme velocities.

Likewise, gravitation certainly does cause a force.
Newton's Law of Universal Gravitation simply say that masses appear to attract masses.
But Einstein General Relativity explains that mass causes spacetime to curve and it is this curving of spacetime that results in the acceleration that causes the force that we call the "force of gravity".

So mass can be said to attract mass but only indirectly via mass "bending" spacetime but note that it not the bending of "space" so much as the bending of "time" that causes this.

Did Newton and Einstein not know that the earth is round when they said earth accelerates towards apple?

Newton Said: Earth also accelerate towards an apple but the acceleration of earth is so tiny to detect as compared to acceleration of an apple

Einstein said: An earth is a non-inertial frame of reference which accelerates upward

Were Newton and Einstein flat earther at the time of formulating their gravity equations?

Did Newton and Einstein not know that the earth is round when they said earth accelerates towards apple?

They both certainly did know that the earth is "round" (almost spherical or a Globe) and rotating.
But what is rotating is not just the body of the earth but all the mass in the system composed of the body of the earth, the atmosphere, the oceans and everything on the earth.

Newton Said: Earth also accelerate towards an apple but the acceleration of earth is so tiny to detect as compared to acceleration of an apple

Yes, the apple with a mass of 0.1 kg would accelerate towards the earth at very close to 9.8 m/s² and
the earth with a mass of 5.9736 x 10²⁴ kg would accelerate towards the apple at very close to (9.8 x 0.1/5.9736 x 10²⁴) m/s² or near enough to zero.

Einstein said: An earth is a non-inertial frame of reference which accelerates upward

No! Einstein did not say that the earth "accelerates upward". The "acceleration" in General Relativity is not acceleration in the position in space but in curved spacetime.

But for most applications the solution using General Relativity is too difficult and is little more inaccurate so most use Newtonian Gravitation.

Were Newton and Einstein flat earther at the time of formulating their gravity equations?

No.
Part of what led Newton to his Law of Universal Gravitation was the realisation that the same effect was involved in:
        objects (maybe the apple ) falling towards earth and
        in keeping the moon flying from flying away from the earth.

Did Newton and Einstein not know that the earth is round when they said earth accelerates towards apple?


Newton Said: Earth also accelerate towards an apple but the acceleration of earth is so tiny to detect as compared to acceleration of an apple

the end part is the key.

Einstein said: An earth is a non-inertial frame of reference which accelerates upward

Were Newton and Einstein flat earther at the time of formulating their gravity equations?

Einstein never said that.