How do you use two Bluetooth speakers to play together on iphone 16

You need speakers that are explicitly designed to be used that way. Consult the manual for your speakers.

Apple Home Pods (and Home Pod Minis) can be linked together in pairs to behave as a single stereo set. That's done in the home app.

bluetooth version 5 is required in terms of support from the speakers, and even not all speakers that claim v5 truely supports it