614.5. A replacement effect doesn’t invoke itself repeatedly and gets only one opportunity for each event.

Example: A player controls two permanents, each with an ability that reads “If a creature you control would deal damage to a creature or player, it deals double that damage to that creature or player instead.” A creature that normally deals 2 damage will deal 8 damage—not just 4, and not an infinite amount.


So in this case the "event" is the orginal coin flip, at that time Thumb A replaces the flip by using two coins, now thumb b goes to replace the orginal flip and sees it is already replaced and replaces it again from thrumb A by flipping 4 coins.  none of this changes the fact it was a single event.

Also chance encounter triggers at your upkeep so if you did enter an infinite flip it would help you win. and frentic sliver does not work with chance encounter the same way frentic efffert does, the sliver has a condition that states it needs to be on the battlefield for the coin to be flipped, so it will only work 1 time, effert doesn't have that condition and can be flipped however many times you want