We propose a fibre-based approach for generation of optical frequency combs (OFC) with the aim of calibration of astronomical spectrographs in the low and medium-resolution range. This approach includes two steps: in the first step, an appropriate state of optical pulses is generated and subsequently moulded in the second step delivering the desired OFC. More precisely, the first step is realised by injection of two continuous-wave (CW) lasers into a conventional single-mode fibre, whereas the second step generates a broad OFC by using the optical solitons generated in step one as initial condition. We investigate the conversion of a bichromatic input wave produced by two initial CW lasers into a train of optical solitons which happens in the fibre used as step one. Especially, we are interested in the soliton content of the pulses created in this fibre. For that, we study different initial conditions (a single cosine-hump, an Akhmediev breather, and a deeply modulated bichromatic wave) by means of Soliton Radiation Beat Analysis and compare the results to draw conclusion about the soliton content of the state generated in the first step. In case of a deeply modulated bichromatic wave, we observed the formation of a collective soliton crystal for low input powers and the appearance of separated solitons for high input powers. An intermediate state showing the features of both, the soliton crystal and the separated solitons, turned out to be most suitable for the generation of OFC for the purpose of calibration of astronomical spectrographs.Optical frequency combs constitute a discrete optical spectrum with lines that are equidistantly positioned. Frequency combs generated in modelocked lasers have been proposed and already successfully tested as calibration sources for high-resolution astronomical spectrographs. However, there is a variety of novel astronomical instruments that operate in the low-and medium resolution range. They also would profit from the deployment of frequency combs as calibration marks. We propose a fibre-based approach for optical frequency comb generation that is specifically suitable for spectrographs with low and medium resolution. This approach consists of two fibres fed with two continuouswave lasers. To be able to generate broadband, stable, and low-noise frequency combs, we need to understand the optical pulse formation in different fibre stages. In this paper, we focus our attention on the pulse build-up in the first fibre stage of the proposed approach and study it numerically by means of the Soliton Radiation Beat Analysis.