You can overlay histograms, or plot them back-to-back or one immediately under the other. So for example, if you overlay before and after for control and then do the same for treatment immediately under it, that would be one way.

You say in comments you're not asked to do 4 histograms, but in fact the way you phrase your question that is exactly what you're asking.

Personally, I'd be tempted to look at one histogram of after minus before for control and one for treatment but that doesn't sound like what you were asked for.

You can overlay histograms, or plot them back-to-back or one immediately under the other. So for example, if you overlay before and after for control and then do the same for treatment immediately under it, that would be one way.

(You say in comments you're not asked to do 4 histograms, but in fact the way you phrase your question it seems like that is what you're asking.)

Personally, I'd be tempted to look at one histogram of after minus before for control and one for treatment but that doesn't sound like what you were asked for.