Thanks for asking guys but others here may be able to help more than me since I haven't trained a 400m person in quite a while. Having said that, I think that both of you are now in the range when the best gains will ultimately be made through speed development. Your question however was for August and September (where the focus is slightly different), so I'd probably do a workout like this:
*Everyday preceded by general warmup, static and dynamic flexibility, sprint or mobility drills, and either a lunge series or hurdle flexibility.
Monday: 2 x (250m, 200m, 150m) w/ 3' / 5' rest; followed by weights (cleans, squats, etc) and jumps.
Tuesday: Extended warmup, tempo (i.e- 8 x 150m on grass at 80% with 45' rest) followed by general strength / medball / bodybuilding.
Wednesday: 2 x (20m, 30m, 40m); then 3 x 90m followed by weights and jumps.
Thursday: Something similar to Tuesday
Friday: 10 x 30m resisted runs or hills followed by weights and jumps.
Saturday: 5 x 300m w/ 2' rest followed by general strength and / or bodybuilding.
There's a big time learning curve to running the 800m so if that's what you want to do I'm sure you'll see big drops as you better learn how to run it.