In Jain cosmology, Siddhashila is the realm where liberated souls reside after attaining kevala-jnana. The 24 Tirthankaras of the current cosmic age have their liberated status (and reside) there after their own enlightenment.

There are no “Tirthankaras of the next cosmic age” currently living or already stationed somewhere on earth or in Siddhashila. The Tirthankaras of the next cycle will be born in that future ascending cycle only after the present cycle concludes; until their births and subsequent enlightenment, they do not exist as known Tirthankaras in this time.

In short:

• The 24 Tirthankaras of the current cycle are in Siddhashila after their enlightenment.
• The Tirthankaras of the next cycle will arise in the future; they are not present now, either on earth or in Siddhashila, until they attain enlightenment in that future cycle.

Note: If you’d like, I can point you to a Jain scripture-based articulation of cycles and Tirthankaras in the Digambara and Śvetāmbara traditions, with careful distinctions noted.