I play Imperial Decree, can I use my 2 Might unit to kill a 2M backline unit and 4M unit of opponent?

No — you can't kill both. Your 2 Might only reaches the 4M unit, so the 2M Backline unit survives untouched.

Damage assignment requires lethal damage to one unit before moving to the next (Rule 465.2.c.3), and Backline makes the 2M unit an invalid target until every non-Backline defender has lethal damage assigned (Rule 826.4.b). Your 2 points must all go to the 4M unit — not enough to kill it normally, but Imperial Decree's trigger kills it the moment it takes damage. The Backline unit takes 0 damage and lives.

Meanwhile the defenders assign 6 back, so your 2M unit dies too. To use Decree against Backline, you need to attack in separate showdowns, one unit at a time (see FAQ 5012).