If you look closely E. flanaganii should make bisexual flowers (at least mine does, and it's self fertile). You can certainly transfer the pollen to your polygonas and see what happens. Probably nothing if I had to guess. I've tried pollinating Euphorbias that are not similar to each other and not much has come of it. You can try to collect any fruit that appears on your flanaganii plant, and grow that species from seed, which is not particularly hard.