When there are two MicroBits in the simulator, they fight over 1 screen.

[TheVogels]

If you create another MicroBit by sending a radio signal, there will still only be one screen and the MicroBits will take the inputs from that one display and fight on what to display on it.

[thomasjball]

For now, you can turn off second mbit simulator: https://makecode.microbit.org/_C2Jg7JHLa65A

[thomasjball]

One solution is to bind the display sim to the first mbit sim, which requires exposing sim identity over the iframe protocol.