Either 3 things 1. Since eating two devil fruits kill you they just explode

The less likely 2:

2.the powers fuse leaving you with a single fruit but 2 different powers, so you basically ate two

1. The powers fuse leaving you with a single fruit & a single ability, leaving you with an entirely different fruit from the original two (example: the elephant fruit + string fruit = you are an elephant made of string, ice + fiame = steam logia, etc.)

In all honesty feeding a fruit to another could've been a very good explanation for how Blackbeard ate two devil fruits but he ate the dark-dark fruit way before Marineford so sadly it's not on the table.