They are both female, right?

The aspirin method I mentioned is all about mixing the uncoated aspirin in the water, and adding that to the soil. I've never tried spraying on just a branch. However...If you want to be sure and preserve the genetics, I'd first root a few clones.
If you haven't done this aspirin method before, I would strongly request you practice it with a strain you have back-ups for.

1 uncoated aspirin per gallon. Water with this as if it's a regular watering. Next regularly scheduled watering/feeding...repeat. Do this a total of 3 waterings or feedings (nutes ok, if scheduled) in a row. Do not drown your ladies.

If you must, you could try the aspirin on one of them asap, grow them out till the male parts produce, (they start showing in approx 2-3 weeks) then collect the pollen when the sacks are about to open. Once the sack start to seriously split, it's ok to harvest. Once dry, the sack will release the pollen with a couple of gentle taps. 5 to 10 of the sacks should do, depending on how much pollinating you want to do later.
After this point it's up to you about continuing the grow with this plant, or cut it way back, and re-veg.
If you continue the grow (as by now mom is nearing the final stages) you may end up with lil tiny popcorn seeds that are underdeveloped, a bitch to clean from your smoke and they explode in the pipe. (no matter how careful, some seeds will occour on this plant)
Anyway, after the pollen has been collected, you can store it till either the clones or the re-vegged moms are flowering. Pollinate lower branches (personal habit) as usual.
Seeds usually take from 6-10 weeks depending on strain and the enviornment. Underdeveloped seeds suck, give 'em enough time to mature.

HTH...Rusty

p.s. please remember that you've added this aspirin to this soil, as it may affect you next grow, if used again.