Session variables would of course be the simplest way, however there could be two other possibilities.
One is to use a service to store the value on the platform (in an infotable property on some thing or on the user extension). Pass in the current infotable and new values and it can combine them and return the new infotable.
Second would be a custom widget extension that includes the infotable as a property and accepts a new infotable that would append the values.
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