Alas, it soon became clear that the Asuryani of Biel-Tan fought not to defeat servants of the Dark Gods, but to humble all who barred the Asuryani path to re-ascension. With every success, Biel-Tan grew more confident, and before long, they no longer limited themselves to the destruction of Chaos. Instead, Biel-Tan began to loose its fury against any alien-occupied planet the Aeldari had once claimed.
They took no pleasure in their victories, seeing them merely as the righteous eradication of vermin, growing ever more bitter each time they encountered an Aeldari world in need of cleansing.
Before long, the pact between Biel-Tan and Iyanden weakened and ran its course. Biel-Tan came to resent that Iyanden did not share its outrage at the primitives who squatted upon ancient Aeldari worlds, and interpreted Iyanden's refusal to engage in such battles as an unwillingness to spill blood alongside its ally.
By contrast, the Iyanden Asuryani saw little value in squandering strength on worlds they could not afford to re-colonise; nor did they hate the aliens so deeply as their Biel-Tan kin -- indeed, the Asuryani of Iyanden pitied many of the aliens for their shortcomings. Though the alliance was never officially dissolved, joint campaigns between the two craftworlds became ever more infrequent until, one day, communications ceased altogether.
This is how the Bromance broke up in Canon.
I could see it enduring longer as Biel-Tan will have greater motivation to remain focused on Chaos this time around.