As someone into this, you have to find men who are respectful enough to stop if things are moving too fast or you're not lubed up enough yet, and that's difficult for a bunch of different but interrelated reasons, one of them just being that finding multiple trustworthy people is just statistically less likely than finding one trustworthy person even before you factor in gender dynamics.

I've noticed even the most sexually respectful men have at least a tiny pain kink, even if it's just a little bit of an "oof" or squeak from going in too hard or fast because it makes them feel like their dick is bigger. 90% of the time that's not really a problem (and sometimes you really do just want to get fucking pounded and feel that stretch so it works out for everybody) but a first DP really needs to be taken slow. And that's even just from a logistics standpoint and even if you've practiced with dildos because you can put a dildo at any angle you damn well please but a dick is attached to a wholeass person that has their own comfort needs including what angles can be held and for how long.

So you need to find men you trust enough to not get carried away and be constantly checking in even nonverbally that your body is relaxed and you're not wincing or moaning in the wrong way despite the fact that they're doing this absolutely wild and taboo act and it's hard to find one guy like that, and generally men that care that much about your wellbeing are also the men who are staying around for a long-term relationship, and THOSE men are almost to a man monogamous. So you can almost definitely find one guy that's willing to go slow enough for anal to be enjoyable, or to insert a dildo into your vagina or anus while also fucking you but the guys who are respectful enough for that to actually be enjoyable are rarely the ones who want to share. Last time I was even in a threeway it was the dude that wound up getting doubled so like.