Is it possible to use the laptop's audio via headphone jack and Bluetooth speakers at the same time?
Yes.

You can do this by aggregating your audio devices.

This is a function that is built into macOS; you don't need to purchase or install any additional software.

Under MIDI Audio Setup (under Applications -> Utilities), you create a new "virtual" device that aggregates audio devices:




While the pictures posted are for an input device, you can do this with output devices as well; Bluetooth, USB, and physical.

Once you create your aggregated output device of both the speakers and BT speakers, you set your application to use that device and sound will play from both devices.