MTG - Cards that share names stack abilities?
No. This does not work that way, The first Creature has entered the battlefield and triggered the ability. It does not then enter the battlefield a second time if you play a second Aether Poisoner. In this situation the only thing the cards share is a name. The ability is exclusive to the card it is read on at the time unless it states otherwise.

In your second example Manna abilities are exclusive to each card so you would have to pay the cost twice - Once for each copy of the card in play. That's provided that you wanted to give both of them flying, you can pay the cost once and give one of them flying.