Hi,
we considered concurrency/fostering to adopt right from the start, and I feel that if you have BC (we do) and are considering concurrency/FtA, you have to be very careful regarding what you say to your BC. Obviously it depends on their age, but generally speaking the stresses resulting from all the uncertainties are huge, and you would want to protect your BC from that.
We told our DS (BC, was 3yo when we started out) that some people can't look after their children due to grown-up-problems, and then somebody else has to look after them; and that we were thinking of doing that. That's how we then explained the prep course and the assessing SW visit - that it was about learning about looking after a child or baby. Further down the line we introduced the thought that sometimes the baby's parent(s) managed to sort out their grown-up-problems and could then look after the baby again; but sometimes not, so that baby would have to stay and grow up somewhere else.
This hasn't been without it's problems but I still feel much better than the other way round. Basically we have been talking 'fostering' rather than 'adoption'. Talking 'adoption', we would have done talking how families are made in various different ways and stressing how the new child would be our 'real' son or daughter, and his 'real' (or forever) brother or sister. But these are conversations that we have left for until matching panel is done and dusted and signed. We have been avoiding speaking about 'new sibling' but rather have been saying 'new baby'. If we get to adopt baby, who we have had placement of for a while now, I'm confident that DS will be able to deal with the 'upgrade' and change in status. But if we had been talking about 'forever sibling' and then baby ended up going elsewhere, I don't know how we could explain that to DS.
So short point of long post is, if you're a prospective adopter and in any way open to/considering concurrency or FtA, and have BC, you need to think very carefully (and early on) about what you say to your BC.
Also, you will have to accept (I mean the prospective adopters that you will be talking to) that your BC will get at least a glimpse into a whole reality that you might have wanted to shield them from. A reality in which some children can't live with their (birth) parents. This can be quite frightening to your BC, who have probably until then never even begun to doubt that you as their parent will always be there for them.
Further, I'd say that as a prospective adopter with BC, you have to be absolutely clear that you are doing it for your own sake. Not for your BC. Your BC and AC MAY get along perfectly well and adore each other; but they may just as well not. And at least in the beginning, it WILL be very hard on the BC. Maybe not just in the beginning.You don't want to end up regretting adopting/resenting either of your children, if they end up not having the kind of sibling relationship you were hoping for.
Good luck - I wish we had had someone like you speaking at our prep group!