Strider Harness
Artifact — Equipment
Equipped creature gets +1/+1 and has haste.
Equip 
(: Attach to target creature you control. Equip only as a sorcery.)
Each journey begins with a single step—and sometimes ends with that single step as well.
Matt Stewart
Scars of Mirrodin (SOM)
• #207Rulings
If a creature enters the battlefield under your control and gains haste, but then loses it before attacking, it won’t be able to attack that turn. This means that you can’t use one Strider Harness to allow two new creatures to attack in the same turn.
(wotc • 2018-01-19)
Rulings
If a creature enters the battlefield under your control and gains haste, but then loses it before attacking, it won’t be able to attack that turn. This means that you can’t use one Strider Harness to allow two new creatures to attack in the same turn.
(wotc • 2018-01-19)