Most cartoonists (and I've been reading comics for a long time) when forced to deal with a similar situation will put the first spoken words at the top with the arrow pointed to the correct speaker (present in panel or not), then put the second spoken words below. Of course, Trudeau painted himself into a corner somewhat with the whole no-lines-around-dialogue thing he decided to use. Of course, he'd have to offset to the left or right to deal with the "arrow"s, so that could help to separate the lines of dialog. I'd say he failed here. Then again, I don't read Doonesbury much, and that may be a device he uses a lot that regular readers would get that non-regular readers wouldn't.