Spectral beam combination based on volume Bragg gratings is an effective approach to obtaining high power laser output. In spectral beam combining system, spectral channel spacing will affect the number of non-combined sub-beams and the overall combined output power due to the finite available gain bandwidth. Based on coupled wave theory, a two-channel high power spectral beam combining model is proposed. By appropriately relaxing the requirements for the spectral channel spacing and line-width of sub-beams, the higher combined output power can be obtained but the spectral density does not significantly decrease. In this work, a 2-channel spectral beam combining system is demonstrated to present a 2.5 kW combined power with combining efficiency 85% by employing a transmitting volume Bragg grating. The combining system has a high spectral density of 0.51 kW/nm with 5 nm spectral spacing between channels. The output can keep a good beam quality when the combined power is less than 1 kW, while the significant degradation of combined beam quality occurs when output power is 1.5 kW and is restricted mainly by the dispersion properties and thermal effects of volume Bragg gratings. During this 2-channel beam combining process, no special active cooling measure is used. Interactions between laser radiation and the grating are verified. Thermal absorption of high power laser radiation in the grating will cause the temperature to remarkably increase, resulting in the thermal expansion of the grating period, which leads to the degradations of diffraction efficiency and the spectral selectivity. Research is also focused on the surface distortion, and the results indicate that the thermal-induced wave-front aberrations of the non-combined sub-beams lead to the deterioration of beam quality. Transmitted and diffracted beams experience wave-front aberrations to different degrees, leading to distinct beam deterioration.