Recommendations for transporting twins on the front of a bike are there any child seats that can be used for twins (6 month old)? I don't like the idea of putting them in a trailer as I suspect its easy for cars to not see it. 

Also - special bonus point to anyone who knows of a solution that is compatible with a tag along bike (used by a five year old)!

Edit I am discussing some custom options with local bike mechanic, will report back when I have update.
An alternative to long tail bicycles is long wheelbase bicycles: