Blocker with First Strike and Blocker with Double Strike
This sounds like it is a bug. Since the way the Combat Damage Step works is


Each player decides how their creatures will deal damage
If any creatures have First Strike or Double Strike they deal their damage
State Based Actions are checked, anything that triggers off of damage happens, players get priority
Any creatures with Double Strike or that didn't deal damage in the first Combat Damage Step deal their damage


So what should have happened is what you were expecting, your creatures deal 1 damage each in the first Combat Damage Step, then when SBAs are checked it should have died due to having damage marked on it equal to its toughness.