If two units with Backline Attack a battlefield, what happens with damage assignment?

When multiple units with Backline attack a battlefield simultaneously, the standard rules for combat damage assignment apply with the specific restriction that Backline imposes.

According to Rule 826.4.b:

• Units with Backline are considered invalid assignments for enemy damage until all units without Backline have received lethal damage.
• If you are the defender assigning damage to multiple attackers, and those attackers include two units with Backline (and no other units attacking that are not Backline), you may assign damage to either of them.

Key points for damage assignment:

1. Sequential Assignment (Rule 460.2.c.3): You must assign lethal damage to one unit in full before moving on to the next.
2. Backline Priority (Rule 826.3): Backline effectively means "I must be assigned lethal damage after any other unit... that does not have Backline."
3. Multiple Backline Units (Rule 826.4.b): If the only units present are those with Backline, they are all "eligible" because all units without Backline (of which there are zero) have already been satisfied. Therefore, you may choose to assign lethal damage to either of the Backline units first.

Example: If an opponent attacks with two units, both having Backline, and you are the defender assigning damage:

• You must assign full lethal damage to the first chosen Backline unit before you can assign any damage to the second unit.
• Because both units have Backline, they share the same priority level, and you are free to choose which one to target first.

This is my interpretation based on the Backline keyword rule (Rule 826) and the general combat damage assignment rules (Rule 460.2). No official FAQ exists for this specific interaction.