If I attack with a Stellarcorn and a Twilight Reveler, may I ready the Stellarcorn that attacked alongside the Twilight Reveler?

Yes. Twilight Reveler's "When I attack, ready another friendly unit" can target the Stellacorn Herder that attacked alongside it.

Attack triggers fire when a unit gains the Attacker designation (Rule 383.4.e), and "another friendly unit" simply means a friendly unit other than Twilight Reveler itself — there's no restriction excluding units that also attacked. Since your Stellacorn exhausted to make the Standard Move (Rule 144.2), it's a legal and meaningful target, so it will become Ready when the trigger resolves. If it were somehow already Ready, the ready would simply do nothing (Rule 415.1.c).