There are 2²⁵⁶ possible SHA256 results when hashing the public key

There are 2¹⁶⁰ possible RIPEMD160 results when hashing the SHA256 hash result.

Since there are more SHA256 results than there are RIPEMD160 results, there must be some SHA256 results which, when hashed with RIPEMD160 will provide identical results (this is known as the pigeonhole principal).

Dividing the number of possible SHA256 results by the number of possible RIPEMD160 results, we find that, on average, each RIPEMD160 result can be computed from 2⁹⁶ different SHA256 results. That implies that there is an average of 2⁹⁶ valid private keys for each address computed with RIPEMD160 (that's 1 : 2⁹⁶ ratio, and not the 1:2 ratio that you suggested).

The odds of finding any one of those 2⁹⁶ private keys is vanishingly small. It just isn't going to happen. With nearly 2²⁵⁶ valid private keys,  and only 2⁹⁶ of them being valid for a given address, you have a 0.0000000000000000000000000000000000000000000000684% chance of a collision on each attempt.

That's a probability of 1 / 2¹⁶⁰.