My first prototype used two 555 (actually a 556) to levitate the small polystyrene pieces. The idea is simple, you generate a 40kHz signal with the 555 in astable configuration and feed it to both transducers. Adjust the distance between the transducers and fine tune the frequency.

The second 555 was used in an inverting configuration, that way you can make a self-oscillating H-bridge, increasing the power delivered to the transducers.

This is the circuit I'll be building this time. Rload is actually the transducers and R3 will have a potentiometer to fine tune the frequency.