Head-mounted displays (HMDs) have not previously been combined with flat-panel display systems and it was unknown whether viewing two displays at differing focal plane distances would lead to perceived blurring or visual discomfort. This is now a concern as the Joint Helmet Mounted Cueing System (JHMCS) is integrated with existing flat-panel display systems such as the Mobile Modular Display for Advanced Research and Training (M2DART). The degree of blurring that could occur would be dependent upon observers' depth of focus and the extent to which the two displays vary in focal plane distance. In previous research, we investigated whether blurring occurs when two displays are viewed simultaneously at independently varying focal plane distances. These conditions simulated those of a monocular HMD integrated with the M2DART. The results of that research suggested that blurring due to two differing focal planes was not likely to be a significant issue for the current configuration of the M2DART. We present here two additional experiments that extend these earlier results. In the first experiment, luminance levels were decreased, thus increasing pupil size and decreasing depth of focus and the degree of blurring was measured using psychophysical techniques. In the second experiment, blurring and visual discomfort were examined under more typical viewing conditions: observers performed a task similar to off-bore sight targeting in the M2DART using a monocular HMD. They identified the orientation of an aircraft target presented on the M2DART and a test letter presented on the HMD. Assessments of eyestrain and perceived blur were obtained during the performance of this task. The results of these two experiments indicated that depth of focus should not be an issue for standard-resolution displays and, further, that visual discomfort is not likely to be an issue for the integration of a monocular HMD with the M2DART.