Fire crews were first called to the Kingdom Hall of Jehovah's Witnesses just after 3am.
Firefighters arrived to find the church "well-involved" by fire.
Less than half an hour later, fire crews were called to the Cardboard Cathedral, on Hereford St, after an automatic alarm went off - triggering an alert to fire authorities.
Southern fire communications said the fire at the Cardboard Cathedral was out by the time staff arrived at the scene.
No injuries have been reported at either site.
A fire investigator is to inspect both scenes this morning.
