Dealing with 2 sessions and use of mask

I am doing a seed region analysis for the entire n-back acquisition in two groups of subjects. I intend to finally compare the groups for differences. 1. Each subject had 2 sessions of n-back runs. I am able to get the seed region based FC maps and Z maps for each session separately. But how can I get a single contrast file from the 2 sessions for each subject? 2. Also - when I view the FC z maps as overlays on mricron I see regions outside the brain template (Ch2). Is this because of differences in voxel size (3x3x3 for FC)/ or is it because I did not use any mask while preprocessing? Many thanks in advance, Lena