Whittierider, an interesting name.
Whittier is a city near home.
I have a question for you. Using a handlebar bag and a seat bag that big, attached to the rails of your saddle, do you have any shimmy issues, at higher speeds? I know the late Sheldon Brown used one.
My seat bag is big (aout two gallons' worth of space) but not nearly as big as the ones I showed above. But to get additional water-carrying capability, I put an XLab dual water-bottle holder on the seat rails, and that sits up even higher. Two of the unusually large Zefal Magnum bottles of a quart (32 oz) each, including the drink, bottles, cages, and attachment gizmo, add about five more pounds up high behind the seat. That's five pounds beyond
the weight of the bag and its contents, and up even higher. (There are two more quarts down on the frame for a total of a gallon, or 128 oz.)
It's the Mountain Wedge III seat bag from Jandd Mountaineering that I got for light touring. It has enough room for a few clothes and things so you can hotel-hop, but only about a third as much room as what I showed further up. It has 7.5 litres of space, compared to 24 litres for the biggest one I showed earlier.
I've used a handlebar bag before but not at the same time with this seat bag, so I cannot totally answer your question. But with this bag and two full water bottles, I do get a minor shimmy if I take my hands off the bars. Speed does not seem to affect this shimmy. I feel the shimmy even at slow climbing speeds if I hold the bars with one hand near the stem while using the other hand to drink from a bottle from the frame. Yet at high descent speeds (even in to the mid-50's of mph, and without clamping the top tube with my legs) I feel no tendency to shimmy with my hands on the bars the normal way. When the bottles on the frame are empty, I swap them with the ones behind. What minor tendency there was to shimmy with the full load behind is gone when even just one of the bottles is swapped for an empty one, meaning there's still one full bottle and the full seat bag.
I used a handlebar bag many years ago along with panniers, but had never seen a very big seat bag yet at that time. I did not like the effect the handlebar bag had on the bike's handling, but there was never a shimmy with it. I kind of expect that the bar bag would calm any tendency to shimmy, but I can't tell you for sure. The big seat bag and the water weight up high behind has an effect on the handling, but it was easier to get used to and compensate for and forget about than the effect the handlebar bag had were.
One plus about the setup I have might be that I made an attachment between the seat bag and the extra water bottle holder so the seat bag cannot move side to side. The water bottle holder does not support any of the seat bag's weight vertically, but it does keep the top of it from going side to side. With the bungee cords that go down to the chainstays, the bag is very well behaved (as long as I haven't forgotten to close it!) and I have been very happy with it. The only thing I could wish for is a tad more room, and you'll get lots more if you either get one of the bigger ones above or augment with a handlebar bag.