Oh gosh - you certainly dont want a First Wheels - they are absolutely HUGE! They are the last pushchair I would want on a bus. The quality is rubbish, the seats are small and I would not recommend one at all. In fact its the only pushchair I have owned that I actually refused to take out for a walk :\
The City Select is actually quite neat with 2 seats on - even in its longest state ie 2 seats facing each other, its shorter than a Phil&Teds and other tandems. But its quite heavy to lift up kerbs - and I guess onto a bus, and your nanny might find it hard to wiggle into position once on the bus? It would probably work ok for the two nanny children, but unless you have a small age gap, it probably wont work so well for your siblings to be. I would say, go and have a good look at it, lift it up a kerb or step and see how you get on. I would also take some extra weights - two x 12kgs and see how you fare.
As for the ABC Adventure Buggy - its definitely NOT a london bus pushchair. Absolutely lovely for walking with out in the country, but not one for a city or public transport.
The fundamental issue with wanting your children facing each other when out, is that they will interact in ways you dont wish eg kicking each other as they get older. The City Select and Oyster Max both have limits on the point at which a child would start to kick the other in the face. Fine now, but maybe not in 6-12 months time. Will they not have enough time to interact during the day?
The best tandems realistically to use and push, are the Britax B-Dual, the iCandy Peach Blossom 2, the Babystyle Oyster Max, the Jane Twone and the City Select.
However I am going to rule the Oyster Max out for you on the basis that you have two youngish babies who will want to sleep and the lower seat recline is not good enough for the lower child to nap.
The City Select I have discussed above.
The Jane Twone - would be quite neat for buses - its bigger than it looks though. However access to the lower child is very difficult. I couldn't even wipe my child's nose through the bars. But it would be easy for the bus.
The Britax B-Dual is notoriously wide and whilst it probably gives you the best option in seating positions, you may wish to consider whether the width will be an issue or not.
The iCandy Peach Blossom 2, to be honest is the best tandem out there for two small children. However you will have one underneath the other (my DC used to fight to get the lower seat because she loved it!) and its very expensive as a nanny pushchair - but may well be worth it long term for you personally.
One final one to consider is the side by side narrow, Mountain Buggy Duet. I personally don't like it for siblings because they can poke, prod etc each other, but the seats are very close and it lends itself better to children on the same age. I dont like the way the fabric bunches around a childs head after being sat up after a nap. The fold is also huge, so be aware. But it may well be a better option for your nanny children than a tandem.
I also would hold the thought that, it may better to focus on the here and now, and sell this double when you know your own age gap, and find something specifically to suit at that time. See this pushchair as a rental rather than a long term investment.