Typically, by definition, the plants will not freely hybridize (that's what makes them a species). I'd love to see some hybrids, but likely it would take genetic engineering to do it. Your first cross is more likely I think than the one with tuberosa, being as they are more closely related species, but I have NEVER heard of North American milkweeds hybridizing.

Edit: After posting, I did a google search, and apparently there are a few suspected (and maybe proven) cases of hybridization of NA Asclepiads...including A. syriaca...so, appears i was wrong...but it's still quite rare and unlikely.

The only hybrid I've ever seen is A. exaltata x A. syriaca. I've seen them in two locations about 80 miles apart. I have tried to hand pollinate several different ones but no luck so far. It's hard enough to just get the species to set seed pods on their own.

I have seen pictures of asclepias syriaca x asclepias speciosa. I have also seen pictures of asclepias syriaca x asclepias amplexicaulis

Interspecific hybrids are very difficult. We have tried a dozen or more hybrid pods with the genetically closest species and had failures on all of them.