Yo Vinnie,
My girlfriend likes the idea of a double penetration but doesn't want to have to deal with all of the "other dude being there" weirdness. So we've settled on a dildo to get the same effect. Question though...does the dildo go in the ass or the other way around? I mean which works better. I ask her and she has no idea either. -- Enquiring Mind (by email)

NO, NO NO, NO!!!...WAIT. OH, ER, OH, NEVERMIND. CARRY ON.

Dear ASS FROM A HOLE IN THE GROUND: She has NO idea? I am SURE she has no idea. She's probably not even sure she's GOT an asshole, I mean other than YOU, right? Well, look, it's nobody's fault here but society's. I mean she doesn't want to tell you because it suddenly makes it less "spontaneous." Like discussing putting anything up your ASS would be anything but. But, no never mind, since the issue is not whether she has an idea or not but whether or not YOU can effectively, carefully, and with a sublime degree of sensitivity figure it OUT. Not for HER, but for YOU, you clod. Blow it? And you'll be denied ass access for, um, ever. DON"T blow it and you will be the Ass King for Life. [Generally we hate the "men as mind-reader" school of feminine dudgeon, but in matter sexual, however, do you really want to be instructing the cock sucking through the cock sucking? Probably not, unless you're German or Swiss or something. No. You want them to FIGURE IT OUT. So, same difference here.]

But to your question: everybody's different and so's all we can do is draw on our own experience, which is copious, especially if copious means "lots of hookers." And so what history has shown us is that it's much better to have the dildo be the upper most item on the fuck tree. So if she's on her back, the dildo's in her quim. If she's bent over in front of you, it goes in the ass because in matters of feel, which is what we're talking about here, what FEELS better is frequently just a matter of control. And it's easier for YOU to control that which you can see.

Outside of not switching between ass and quim without a quick wash, we have no cautions.

Plunge ahead, sir!