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