XFCE: How to set different x/ythickness for each separate panel in .gtkrc-2.0?

Question

The question is in the subject. Can it be done? Is there any way to do it other than editing the .gtkrc-2.0 file?

Full story:

I use two panels in my desktop. Both horizontal, one in the top and the other in the bottom.

I was personalizing my theme so that the icons in the top panel looked big, so I had to change 3 to 1 in this part of the Adwaita theme's gtkrc:

style "button" {
    xthickness = 1
    ythickness = 1
}

While this solves the problem for the upper panel, it isn't looking good in the bottom panel. There I need the old configuration i.e. xthickness = 3 ythickness = 3.

I found out that I can change the panels' properties by editing the ~/.gtkrc-2.0 file. For instance:

style "panel"
{
    xthickness = 3
    ythickness = 3
}

widget_class "*Panel*" style "panel"
widget "*Panel*" style "panel"
class "*Panel*" style "panel"

Unfortunately, but each change I make applies to both panels and I can't find a way to set the x/y thickness for each panel separately.

How could this be done? Is there another way other than editing .gtkrc-2.0?