First I'd find out both of their individual donation amounts from showcases, reviews, P411 if you / they have it.
Then I'd ask them (without mentioning a dollar amount) if I should assume that their doubles rate is the total of both their hourly rates from their showcases or if they have a special doubles rate. Don't mention any activities whatsoever. Also check reviews on them - if there are any doubles reviews, the donation listed on those reviews is your answer.
You are still fairly new just going by the date you joined eccie and mentioned that all your experience is with Backpage so far. Do you have any references from ladies here yet? If not you have a double whammy going on - without references you'd still be considered a newbie and not trusted just because of that, and moreover a newbie asking about money, which would be seen as a red flag. If that's the case - and I'm not saying it is, just seemed that way from your question - the newbie issue needs to be addressed first, I think, by seeing a newbie friendly provider who is also reference friendly.
One other thing to keep in mind is how you make contact can also matter. Most ladies aren't going to respond very well to a guy they don't already know asking questions by text. Many will be more comfortable with an eccie PM but probably only if you've built up a bit of a posting history on the forums and especially if you have done some reviews (and you can do reviews on BP ladies, though they won't build your credibility as much as ones on eccie ladies). Best of all in my opinion would be a PM on P411 if you're a member, because you've already been employment verified by P411 and you've paid to be a member so it's less likely you'll be perceived as a time waster. Joining P411 really was the best thing I did as a newbie and certainly opened a lot of doors for me. It's not necessary but it does grease the wheels quite a bit. I find enough value in it that I renewed for a second year even though I have lots of references.
You mentioned "I'm just looking for a double BBBJ" which seems to suggest you might think that could be a different rate than a full fledged double for the same amount of time (?). I don't think that will be the case, if so. Most if not all ladies are going to require their normal rate for their time whether it's wall to wall boinking, double BBBJ, or anything else. You might also thinking that it could be different because it would be a shorter session (half hour?) than a regular double, but my impression is that most ladies aren't going to be interested in a double for less than an hour, regardless of activities. I could be wrong on that - would be interested to hear some providers' perspectives on it.
None of this is intended as a criticism of you, just trying to be helpful.