Steve:

I doubt it, mainly due to the requirement to turn the invoice side ways.

What you could do is print the picking slip on top of the invoice (so that each takes half of an 8.5 x 11 piece of paper printed portrait).

Another option would be to print the picking slip on the back of the invoice (or vice versa). I think this would be the better option as the first option may be too limiting on the number of available detail lines and still limits to one piece of paper (which I'm assuming is the goal).