An example myhdl sim showing two sims running concurrently in different threads with no problem. In such an example, it is necessary to have separate signals, otherwise they *will* step on each other.

myhdl_multithreaded_sims.py

from myhdl import *
import threading

a = Signal(intbv(0)[5:])
b = Signal(intbv(0)[5:])

clock1 = Signal(False)
clock2 = Signal(False)

reset1 = ResetSignal(False, active=True, async=False)
reset2 = ResetSignal(False, active=True, async=False)

lock = threading.Lock()

def clockgen(clock):
    @instance
    def clkgen():

        while True:
            yield delay(10)
            clock.next = not clock

    return clkgen

def stimulus(sig, clock, reset):

    data = [0]
    @always(clock.posedge, reset)
    def stimulus_inst():
        sig.next = data[0]
        data[0] += 1

    return stimulus_inst

def printer(sig, clock, reset):
    
    @always_seq(clock.posedge, reset)
    def printer_inst():
        with lock:
            print threading.current_thread(), now(), sig

    return printer_inst

sim1 = Simulation([clockgen(clock1), stimulus(a, clock1, reset1), 
                   printer(a, clock1, reset1)])

sim2 = Simulation([clockgen(clock2), stimulus(b, clock2, reset2), 
                   printer(b, clock2, reset2)])

sim1_thread = threading.Thread(target=sim1.run, args=(200,))
sim2_thread = threading.Thread(target=sim2.run, args=(300,))

sim1_thread.start()
sim2_thread.start()

sim1_thread.join()
sim2_thread.join()

The output from the above is something like:

<Thread(Thread-1, started 139843183535872)> 10 0
<Thread(Thread-1, started 139843183535872)> 30 0
<Thread(Thread-2, started 139843175143168)> 10 0
<Thread(Thread-1, started 139843183535872)> 50 1
<Thread(Thread-2, started 139843175143168)> 30 0
<Thread(Thread-1, started 139843183535872)> 70 2
<Thread(Thread-2, started 139843175143168)> 50 1
<Thread(Thread-1, started 139843183535872)> 90 3
<Thread(Thread-2, started 139843175143168)> 70 2
<Thread(Thread-1, started 139843183535872)> 110 4
<Thread(Thread-2, started 139843175143168)> 90 3
<Thread(Thread-1, started 139843183535872)> 130 5
<Thread(Thread-2, started 139843175143168)> 110 4
<Thread(Thread-1, started 139843183535872)> 150 6
<Thread(Thread-2, started 139843175143168)> 130 5
<Thread(Thread-1, started 139843183535872)> 170 7
<Thread(Thread-2, started 139843175143168)> 150 6
<Thread(Thread-1, started 139843183535872)> 190 8
<class 'myhdl._SuspendSimulation'>: Simulated 200 timesteps
<Thread(Thread-2, started 139843175143168)> 170 7
<Thread(Thread-2, started 139843175143168)> 190 8
<Thread(Thread-2, started 139843175143168)> 210 9
<Thread(Thread-2, started 139843175143168)> 230 10
<Thread(Thread-2, started 139843175143168)> 250 11
<Thread(Thread-2, started 139843175143168)> 270 12
<Thread(Thread-2, started 139843175143168)> 290 13
<class 'myhdl._SuspendSimulation'>: Simulated 300 timesteps

myhdl 0.9 will just run forever.