First column: snapshots of observations and MPI-AMRVAC simulations taken when CME1 front is located at 10 R_⊙. Second column: snapshots of observations and simulations taken when CME2 front is located at 6 R_⊙ (except (h)). Panels a and b: running difference images taken from EUVI, COR1, and COR2 instruments on board STEREO-B and visualised in JHelioviewer software. Snapshots of the simulated relative density and selected magnetic field lines: panels c and d indicate the double eruption scenario, panels e and f show the stealth eruption scenario, and panels g and h indicate the failed eruption scenario. The parameter Δt represents the time interval from the eruption of CME1, chosen as 21 September 2009 21:00 UT, or 9.6 h simulation time after the start of shearing motions, when the centre of the flux rope crosses 1.5 R_⊙ and enters COR1-B field of view.