This is one of those things that's so obvious I had to trip over it to figure out. I'm not any kind of expert so there may be something that won't pan out here. I'd welcome comments.
Part 1. Getting 2 seasons of crop in a bed at one time.
I use raised beds. This gives me the sides of the beds to work with. I plant specific plants along the sides (the slope) of the bed with the main crop going in the regular flat part. When time comes along to transplant say tomatoes into the pea bed, that's fine because the peas are on the sides. The middle of the bed is vacant. Lettuce, radishes, beets, peas, all can grow on the sides.
Part 2 Saving seed from more than one variety within a type of vegetable.
Brook gave excellent directions for a cover to isolate a vegie and many of us use individual scraps of panty house or whatever for vegies like tomatoes and peppers. For plants that make a seed stalk or cluster of seed stalks the simple expedient to prevent crossing is to cut off the flower stalk of all competing varieties (cept the one you want to save seed from) till the flowers are gone from the first one. Now you do have to stay vigilant. Once the plant is ready to go to seed, it won't just give up. It'll keep trying to send up a stalk to yanked or allowed to go on. But I've found (so far) that basil and lettuce tolerate this practice extremely well.
I recognize that there may be a finite amount of energy in the plant and potentially this process could exhaust the plant before a good crop of seed can mature. I haven't needed to go that long however. So far doesn't seem to be a problem.
Maybe this will help with planning next years garden
I'm a little confused, Chris, by your description of raised beds. What do you mean by the "slope?" And, why is the middle left vacant?
Generally what you're describing is succession planting. Ideal is to have the first crop finish just about the time the second one has to be transplanted. Peas, as you point out, are ideal for this because they finish just in time to plant tomatoes, peppers, and other hot weather crops. Plus the peas have helped fix nitrogen. And, after picking the dried seed, it can serve as a green manure if you just till it in before planting the second crop.
As to your second point, a great idea. Many growers merely pick varieties that come to flower at different times, but I've never been able to plan quite that systematically myself.
I'm a little confused, Chris, by your description of raised beds. What do you mean by the "slope?" And, why is the middle left vacant? >>
I wondered if I had confused everyone. My raised beds are not supported by wood or anything along the sides. The dirt stays there just fine on its own, so I have the bed itself with a H above G of around 6" and the sides of the bed that slope down to the paths between beds this creates a 6" wide 15' long area that can be planted with whatever.
Flowers herbs, lettuce, peas etc bush beans work too. This year I planted the peas along the sides (the slope) of the beds destined for tomatos and peppers. The salad bed was slightly different. Lettuces, radishes and onions were all broadcast along the sides and top of the bed. At this point of the season I've pulled all the lettuces except those for seed. I'd managed to have enough of each variety along the slope so that now that bed is being prepped for squash without affecting the lettuce for seed.
Just because from say Nov to Mar I dont need most of the space so it's easy to plan for this. A time for the grass clippings to break down and the soil basically rests.
I know the really clever gardeners have a way of figuring out when different varieties will go to seed (or mature). Somehow I can't seem to get that. I had a heck of a time this year trying to figure out when the various corn will tassel. Couldn't do it. Decided to just grow the one (for seed) here and the rest at Dad's and forget those for seed :)