Hi,
without having tested it, in the example that you show, I would try the following:
Coming from the table with seven columns you set a visibility condition (on the columns 3 trough 7) so that only the two leftmost columns are left (you need to set the condition on both the COLDEFS and the COL elements).
I think that this is sufficient if the left column has fixed width (I think that there is code in the engine that makes the rightmost column have relative with if no other column has relative width).
If you want to be sure than you make sure that the left column has fixed width and the description column has proportional width (specified in the COLDEF).
If that isn't already the case in the initial layout with the seven columns then you can create two mutually exclusive COLDEF nodes (via visibility condition) for the description column that have either fixed or proportional width (One is used in the case of seven columns the other in the case of two columns).
I hope that I understood the problem correctly and that this helps,
Alex