Auditioning for Handel's Messiah concert as a second soprano, do they even want us seconds?

Generally, a second soprano is a mezzo-soprano. Most choral works do not have a separate vocal line for mezzo-sopranos. As such, mezzo-sopranos can choose either to sing the soprano vocal lines if they are comfortable in doing so, or they can choose to sing the alto vocal lines.

"Lift Up Your Heads" is the only chorus in "The Messiah" in which Handel wrote a vocal line for second sopranos, which is sung by either sopranos or altos.

The highest written note for sopranos in "The Messiah" is the high B-flat above the treble clef. Do audition if you can comfortably sing the soprano part and if you can sing the high B-flat

"Lift Up Your Heads" (No. 33) is partially scored S1 S2 A T B. However, the vast majority of the chorus are straight SATB, and the S part is often up to A and once in a while Bb. So whether you audition to sing soprano would depend upon your range--G and A are within the range of most mezzos (and the Bb is pretty rare).