Longitudinal-mode combustion instabilities in can-annular gas turbine combustion systems are influenced by acoustic interactions between adjacent combustors; these interactions typically occur across an open area between a combustor transition piece and first stage turbine nozzles. Here we show that strongly interacting thermoacoustic modes are excited in two adjacent model gas turbine combustors connected with a cross-talk area aligned normal to the flow direction. The data are analyzed in comparison with the measurement results of a single isolated combustor without the cross-talk area. Our results demonstrate that the dynamics of the multiple combustors are manifested by a push-push, a push-pull, and a bimodal mode, which are induced in response to the difference in equivalence ratios between two combustors, including symmetric and asymmetric conditions. The push-push mode develops due to in-phase interactions between adjacent combustors, leading to the formation of a pressure anti-node in the cross-talk area. By contrast, the push-pull mode is a result of out-of-phase interactions, and forms a pressure node in the cross-talk area. Under marginally asymmetric conditions, the system features bimodal tones with a strong beating oscillation, which results from a combination of closely-spaced push-push and push-pull modes. Mode pairing is the dominant mechanism for the growth of the beating oscillation. The mode selection process is not directly determined by the local flame dynamics, but by the distribution of acoustic energy between the fundamental and higher harmonics in the cross-talk area. When a push-pull mode is initiated in the system, higher harmonic components with a large amplitude begin to emerge, strongly diminishing the fundamental frequency.