Atomization for spray drying of high viscous feed liquids is still a challenging task. For this reason, we investigated the potential of two internal mixing pneumatic atomizers, namely an effervescent atomizer (EA) and an Air-Core-Liquid-Ring (ACLR) atomizer. Both atomizers are characterized by a two-phase flow in the exit orifice. While this can be either a two-phase plug or annular flow in case of the EA geometry, the ACLR atomizer enforces annular flow conditions. In this study, spraying experiments were conducted at liquid viscosities between 0.12 and 0.69 Pa∙s. The investigations were performed at a constant liquid flow rate of 20 L/h and gas pressures from 0.3 to 0.9 MPa. Besides the commonly used correlation between Gas-to-Liquid-Ratio (GLR) and time-averaged Sauter mean diameters ((SMD) ̅), we analyzed in-depth the time dependent fluctuation of SMDs, as steady atomization is crucial for spray drying applications. We can conclude that due to strong fluctuations of the SMDs the EA is not suitable for the aimed application in spray drying of high viscous feed liquids. In contrast, the ACLR atomizer is a very promising nozzle for spray drying applications as it delivers much better performance and steadiness also at high liquid viscosities.