Can the 2 yo yo strollers be connected side by side or it only can be connected train style?
They cannot be connected side by side, no - at least not with this Yoyo Connect extension. The Yoyo Connect only works as a tandem.
If you needed two side-by-side Yoyos, you would need to buy two regular Yoyos and the universal buggy connectors, e.g. on Amazon. Also, there is the Bumprider Connect that offers the side-by-side magnetic connection, just it is expensive.