if equipped with two handed weapons like longbow, flail etc, can you also equip the bucker shield and gets its effects ?

I'd say no. A buckler still requires a free hand to wield properly, it isn't just a piece of armor strapped on. Also, part of the defense a shield provides assumes that you are getting it between you and an incoming hit. Even if you could strap a buckler on, you wouldn't be free to move it to intercept a blow while wielding a two-handed weapon. You may as well allow someone to strap a tower shield on their back and pivot in place then. That's clearly a trade off you make when you a two-handed weapon; knowing that you can't benefit from a shield of any type.

Yeah, a buckler isn't strapped onto the forearm, it's gripped in the fist; the shield is actually in front of your hand, not alongside your arm. It's positioned so that you can punch with it. If you open your fist, the buckler falls on the ground.