They can be only encountered in The Chest of Chapter 6.
They behave identical to Conjoined Fatties but with a higher health and different attack method. Instead of spitting a line of creep, they spit a homing IPECAC-like bullet at Isaac.
Blue Conjoined Fatties usually come 4-6 in pack from one room.
They won't turn into a regular Fatty upon death, unlike the regular Conjoined Fatty.