A fantasy that they could harness the fanatical energy of Islamism and use it to empower themselves. The Cairo-Khartoum cabal was the first to suffer from this delusion, and we’ve all been living with the consequences for nearly a hundred years, and it keeps getting worse.
At some point, these two dictatorships are going to come into conflict. Islam regards itself as a political as well as a religious system. Communism regards the religious and the political as one and the same, which is why its political system suffices (as we see with their persecution of Chinese Christians, now that Pope Marxist I has betrayed them).
They’re just allies of convenience at present, both hating the US and seeing us as the main obstacle to their (conflicting) plans.