Merging vertically is not possible; however, I believe you can achieve the same effect by inserting a nested table inside of text boxes adjacent to the ones you want extra tall.
Let's assume you have six values to display, columns A through F of a standard table. But let's further assume that values A and B need extra vertical space because the data and values C through F fit fine into a regularly sized textbox. So that means you want your table to have two columns that are one textbox tall and two columns that are two text boxes tall.
To achieve this effect, create a table that is one row tall and three columns wide (disregarding the header row). In the first text box put value A and in the second text box put value B. In the third text box insert a table. (Yes, a table.) Delete the header row of the nested table and also one of the columns. In the third column of the primary table you now have a nested table with one row and two columns. Add a new row to the nested table that is inside the grouping (i.e. another detail row, not a header or footer row). Now the nested table is two rows high and two columns wide, and you can put into the text boxes of the nested table each of the four remaining values, C through F, in whatever placement you see fit.
This technique works because the scope of the nested table is identical to the scope of the text box in which it is nested.