If your common project is sufficiently large, you can agree in advance to split it into two publications. You submit paper #1 under your real name and your coauthor submit paper #2 under his real name. Each paper cites the other paper (I hope this is not a problem with the authorities in your country to cite an author from a hostile country). This way, each of you receives proper credit and even a citation.

If your papers become sufficiently important, it is likely that future researchers will cite both papers, so you won't lose anything in terms of citation ratings.

A disadvantage of this approach is that MAY be considered "salami publishing", defined as: "the situation that one study is split into several parts and submitted to two or more journals". This may be considered unethical. However, if your project is sufficiently substantial, it may be too long for a single paper anyway, so the splitting may be justified from an academic perspective.

Just for fun, I would also use ideas from Livings' answer: "you could embed a secret that only you know, e.g. a hash of a secret text, a product of two big primes or similar. Extra points for embedding those in a way that no one recognizes"

If your common project is sufficiently large, you can agree in advance to split it into two publications. You submit paper #1 under your real name and your coauthor submit paper #2 under his real name. Each paper cites the other paper (I hope this is not a problem with the authorities in your country to cite an author from a hostile country). This way, each of you receives proper credit and even a citation.

If your papers become sufficiently important, it is likely that future researchers will cite both papers, so you won't lose anything in terms of citation ratings.

A disadvantage of this approach is that MAY be considered "salami publishing", defined as: "the situation that one study is split into several parts and submitted to two or more journals". This may be considered unethical. However, if your project is sufficiently substantial, it may be too long for a single paper anyway, so the splitting may be justified from an academic perspective. As an example, see these two papers by Gul and Stacchetti, which are on the same topic and were published in the same journal in two consecutive years. Each paper cites the other "companion paper".

Just for fun, I would also use ideas from Livings' answer: "you could embed a secret that only you know, e.g. a hash of a secret text, a product of two big primes or similar. Extra points for embedding those in a way that no one recognizes"