Growing 2 especies and not crossing their pollen

This would be easier indoors. Outdoors is much trickier.

A plastic divider could work, but you'd have to make sure it was absolutely air-tight, which would probably be quite hard to do. An airtight greenhouse would not be good for the plants inside anyway, as they will need good airflow to replace the CO2 they take from the air and allow them to photosynthesise properly.

Starting the strains a month apart will not help, as plants flower according to photoperiod, not age. Younger and older plants will still start blooming at around the same time - when the day gets short enough to trigger flowering.

One option I can think of would be to plant the two strains a long distance apart - hundreds of metres if possible. This would still not guarantee that there would be no cross-pollination, but it would reduce the chances.

The other option would be to plant everything together, with clear markings as to which plants belong to which strain. When flowering begins, you'll usually be able to spot the males first, and at this point you should kill off all but the biggest/healthiest/earliest-flowering male from each strain.
The surviving two males should then be dug out of the ground and taken far away from the females. Indoors, under a 12/12 light cycle would be a good place (a few 120cm fluorescent tubes would be fine for this kind of indoor grow). In the 12/12 cycle, the males will flower quickly and you can collect pollen from each, store it in the freezer until the females are ready to receive it, then carefully apply small amounts the appropriate pollen to the appropriate bud with a small paintbrush.

As far as preserving strain integrity in the f2 generation goes, cross-pollination might not make a huge amount of difference.
If the two strains in question are hybrids (which is almost certain), then the f2 offspring of each are probably going to display a wide range of different traits and forms. Unless they're true-breeding strains, then the f2 generation may not bear much resemblance to the f1 generation, even if pollination is kept strictly within each strain.

Having two different pollen fathers in the mix will likely increase variation, but this is not necessarily a bad thing.

If you do go to the effort of separating the males and manually applying the pollen to the females flowering outdoors, it's not a bad idea to try manually cross-pollinating, just to see what happens when you mix the strains.
You could apply most of the pollen from each male to females from the same strain, and reserve a bit of pollen from each to put onto a female from the other strain, resulting in four types of seed - two sets of f2 seeds and two sets of new f1 hybrid seeds.
When making a hybrid of two strains, which strains supplies the male makes a difference. That is, pollen from strain A applied to strain B will gernally make a differnt hybrid to pollen from strain B being applied to strain A.

Hi,

sow your seeds rather late in the season so the plants will not get huge. Look at them each day. When the first pollen sack is about to open then pick it off, open it with your fingernails and smear the dusty crumble onto a pistil. Cull the plant and bury it far away.

Good luck with your farm!