Ekadashi's paper of the first atomistic simulations on the T1T1 → T2S0 triplet fusion is accepted for publication on J. Phys. Chem. Lett. In this work, simulated the fusion for a model intramolecular singlet fission (iSF) chromophores that feature E(T2) < 2E(T1) and hence a thermodynamically favourable triplet fusion. We found that despite this thermodynamics driving force, the fusion is hindered by the weak vibronic coupling between the T1T1 and T2S0 states. The weak coupling is traced to an T1→S0 intersystem crossing (ISC) hidden under the veil of internal conversion of the triplet fusion. We pointed out that all iSF chromophores feature thermodynamically favourable triplet fusion. The hidden ISC saves iSF chromophores from this undesired thermodynamics sink. The intrinsic incompatibility between iSF chromophores and the general requirement of E(T2) > 2E(T1) is resolved. The hidden ISC also applies to regular non-intramolecular SF chromophores. Therefore, the E(T2) > 2E(T1) requirement is relaxed for all SF chromophores, intramolecular or not. One hurdle for searching for and designing SF chromophores is removed.