SSI Web software doesn't automatically let you do that within the same CBC exercise. The trick is to make more than one CBC exercise within your SSI Web study (where the second exercise has a different number of concepts per task), then to move the CBC tasks around within that questionnaire so that the flow of the CBC tasks across the two exercises is how you want them (for example, you could intersperse choice tasks for the two different exercises within the same series of choice questions).
But, SSI Web will still treat these two choice exercises independently during analysis. If you want to integrate the two exercises within a single utility analysis, you'll need to export the data from the two exercises to separate .CHS or .CHO files (whichever format you prefer). Then, you'll need to use your own data processing tools to manipulate those files and stack them per respondent so that all the choice tasks are collected within each respondent record.
