I think it's a bad idea. It seems like it's using kids to have some kind of concrete bond between you guys, but it completely ignores said kid in the process.
Are you all living together? If you were, then I would understand something along the line of "let's stop using birth control, and whoever gets pregnant, we'll raise the kid together and consider each of us a parent". And of course in a case like that you might not even know who is the father, or care for that matter.
Still, if you don't live together, where would the kid live? Would each kid live with their mom and be raised by the non-biological father, with biological ties to the other?
It all sounds very messy and I would say, if you want a strong bond, try something that won't involve someone else for the entirety of their lives. If you don't live together, think about moving together, for instance. Of I don't know, make joint purchases. Something that binds you together but, if things go wrong, will only be a mess for the four of you, without catching kids in the middle of it.
And well, since you don't feel that it's a good idea, then definitely, don't do it. Any child should ideally be planned and really wanted, because it's hard enough when it's a child you seriously want, it's all the harder if you're not into the idea. And you certainly don't want to regret having a kid, that would suck for everyone involved.
I can see an idea like this one work in a completely different situation, but I don't think you've been together long enough to be sure you're past the NRE and in the right mind to raise kids.
Also, I would suggest not having two at once (seems like the plan was for both women to be pregnant, then later both of them to be pregnant again, each time at the same time). I'd say if you're lucky enough to have more people to take care of kids, don't "ruin" that advantage by having more kids simultaneously. You might find that four people for one kid might already be hard on all of you.
Me: 31F, straight
Seamus: My husband, 32M, straight
Dragon: My boyfriend, 29M, pansexual, married to Fox
Fox: My boyfriend, 29M, homoflexible, married to Dragon