Fish Tank Virtual Reality (FTVR) displays create a compelling 3D effect with the motion parallax cue using the head-coupled perspective. While the head-coupled viewpoint control provides natural visuomotor coupling, the motion parallax cue has been found to be underutilized with minimal head motion detected when manual input becomes available to users. We investigate whether users can effectively use head-coupling in conjunction with manual input in a mental rotation task involving inspection and comparison of a pair of 3D cubes. We found that participants managed to incorporate the head-coupled viewpoint control with the manual touch input in the task. They used the touch input as the primary input and the head as the secondary input with the input ratio of 4.2:1. The combined input approach appears to be sequential with only 8.63% duration when the head and manual input are co-activated. The result of this study provides insights for designing head-coupled interactions in many 3D interactive applications.