The spell Spirit Guardians is a 15' AoE spell with a range of Self, and it is a Concentration spell that lasts for 10 minutes. (PHB, 278)
Find Steed includes the following: (PHB, 240)
While mounted on your steed, you can make any spell you cast that targets only you also target your steed.
Regarding spellcasting range, the PHB reads on page 201:
Other spells, such as the shield spell, affect only you. These spells have a range of self.
When multiple spells are in effect, the PHB says on page 206:
Combining Magical Effects
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect—such as the highest bonus—from those castings applies while their durations overlap.
For example, if two clerics cast bless on the same target, that character gains the spell’s benefit only once; he or she doesn’t get to roll two bonus dice.
Suppose I am mounted on my steed and I cast an AoE Concentration spell that targets only me, such as Spirit Guardians, and I choose to have it target my steed as well. It looks like creatures surrounding me will notice no difference as the spells effects do not combine, correct?
However, having cast this spell, if I dismount my steed and send it off to another part of the battlefield where our AoEs do not overlap, does the spell still stay in effect for my steed for its duration as long as I hold concentration?