Micro tandem bi-copters are capable of passing through narrow gaps owing to their particular slender shape. However, the introduction of the tilting servo motors leads to a non-minimum phase roll dynamics, which affects their flight stability when exploring environments with unpredictable disturbances. In this letter, we propose and design a re-configurable aerial platform consisting of two modular bi-copters with an undocking mechanism. In combined configuration, a crossover docking approach is employed to compensate for the poor stability in their servo-controlled attitude of each bi-copter. In bi-copter configuration, the minimum size (equal to ideal passable gap's width) of the system was reduced by 58% through mid-air separation. In detail, to compare the attitude response of the two configurations, a dynamic model considering servo response and non-minimum phase is established and simulated, and flying poking experiments were also conducted on them respectively. On the other hand, the performance of single bi-copter including trajectory tracking and passing through narrow gaps was demonstrated through flight tests. Finally, the feasibility of the undocking mechanism was verified by mid-air separation experiments. The proposed system is promising to be applied in scenarios containing both complex perturbations and confined spaces, while also having the potential to improve exploration efficiency through collaborative work.