This answer requires you to create an extra column in your data table but it might be helpful for you.
Have you tried the Statistical - Col Mean function under column formulas? It has a little ^ symbol which means if you add a comma after the name of the column you wish to summarise you can add by variables. Assuming the column you are summarising is called Y.
Col Mean(:Y, :Item)/2
Would give you the summary statistic you desired. Then in tabulate the mean, min or max or this column by Item will give the result you require and you can edit the column text as desired.
I often use this in combination with if statements in my main table so each summary value for each group only appears once in each column. It is the JMP equivalent of SUMIF, AVERAGEIF, COUNTIF, etc.