I really don't like the bottom figure. Any tips?
I really don't see anything wrong with the bottom figure. Maybe you don't like it because it's in that aquard almost-on-the-page spot at the bottom.. having him clearly going off the page, or leaving some space at the bottom would fix that easily though... I know the back of the head isn't finnished, but it might be nice to get a referance or something to look at for that, can be confusing. Just noticed something.. might be beneficial to either make the lower figure's left arm a little larger, or make the twist in the shoulders a little more obvious.. Just nit-pickie shit, over all, I'd say it's a good drawing.
I agree with ChromaticCodex, I think it looks good the way it is now, but I think that when you start drawing the head, you should add a neck. Right now it seems as though his head is just sitting on the shoulders giving the look of a "hunched back" and not someone actually leaning over with is head forward like in the top picture. In reality the neck is probably pointing outward, and up a bit...and I think we need to see more of it. Hope that helps!
Any of the fixes mentioned ought to work. Might i also suggest that completing the roughed out drawing first, before starting to render up would be useful. You then have the freedom to pick and change at your will without having to erase carefully detailed shading.
if you really aren't happy with it, a change in perspective might be what you need. it means redrawing (and nobody ever wants to do that..) but the composition of a full front figure versus a full back figure basically on the same vertical is a bit more static than if you were to swivel the ground plane around at an angle so that maybe you see caesar's back view from something of a 3/4 or less, and brutus from the opposite 3/4. You might get more of the feeling of brutus in the background and caesar coming forward, instead of as is, they kind of blend into eachother's space.
k4 had a good suggestion with the rough.. you could do several thumbnails with simple silhouetted figures to help you get a feel for what composition works best. Like i've heard before, if you completely black out the figures and the viewer can still understand what's going on, then you've got it right.