We have siblings and while our local support network is small, the few we have, provide a good deal of help and it's needed often. It is the hardest thing I've ever done. The times we need a grandparent to pop over for a couple of hours is increasing. With school visits now and nursery taster sessions for the other, birthday parties, activities etc. our children need one of us to attend on our own with them, to have our full attention for support, because their security and lack of confidence just needs it (and we have children with very minimal extra needs). Fine on a weekend but it's often while DH is at work and so it's a case of asking grandparents to help out. Also it's just the sheer exhaustion of it all, there is no break, if DH is here then if we're not together as a family we are splitting them up to give them their much needed one to one time. My body physically hurt everyday for months because you've got a heavy toddler in your arms every second of everyday, because if one isn't in your arms, the other is and your body hasn't built up from the baby stage. There's then the sibling rivalry, the wanting me to themselves all the time, the fighting over toys, of course you'd get that with birth siblings but their reactions are different e.g. ours will tantrum where they physically harm themselves for attention as that's the extremes they've learned in their earlier lives. Like many adopted children they've also had few boundaries put in place prior to fc placement so keeping an eye on 2 toddlers is even more exhausting than most of my friends find it. All this means a good support network is so so vital. Adopting one child is not easy and yet having gone from 0 to 2, during the small amount of time one has been at nursery, it FEELS like a piece of cake! My prior working life had prepared me quite well for the demands, energy and hours of siblings, something that was noted at approval panel as a reason we were approved for siblings, but nothing could really prepare you for it, Ive never been so tired in my life and I once worked 60 hours straight with only 5 hours break (to sleep)!
I am going back part time after a year but I'd switched jobs prior to adoption to allow me to work hard while I'm at work but not bring my work home with me, as my other job would have required, with a high level of stress, so the return to work thing I think also depends on what job you'll be returning to.
I've met a couple of people on training sessions who've said they wanted to adopt siblings but their SW 'persuaded' them to adopt one child instead. I wonder if your SW hasn't told you before because she/he too thought she may be able to 'persuade' you and now realising she hasn't, after all this time has said something. If it were me I'd have preferred the honesty from the start too tbh. I will say this though, we have the most perfect match and with all it takes I wouldn't change our decision to adopt siblings, it's the hardest but best thing we've ever done, however, while we were adamant we only wanted to adopt siblings and wouldn't be happy any other way, on the occasions where our eldest has been at nursery and we've experienced just being with one child, I've realised had we only be allowed to adopt one child we'd have been just as happy.
If you feel like you can't move forward without a second opinion, get one, for piece of mind. I think though that they are very cautious when approving adopters for siblings, so they're not saying you can't do it, they're just saying if there's any risk that puts a doubt in their mind they won't do it and in your case it's probably just support network. Much like single adopters need an even more solid support network than most (how you single adopters do it I don't know btw!), I think that is the same for those adopting siblings.
It also must be hard because if you've experienced infertility it's always about being told no, isn't it and you kind of lose control of everything, it's all in a doctor's hands or a SW's and this is just another decision taken out of your hands that alters how you saw your future once again and it's hard However, if a second opinion doesn't change the situation for you, I guarantee in a few weeks, months or a year when you have one LO placed with you, you'll be too happy.....and too exhausted, to even think about this decision being made for you as hard as it is now.