Martial Arts and number of weapons

How does wielding weapons affect the number of attack bars for a Martial Artist?

As I understand it, if you wield weapons with "Martial Arts for combined attack", you can get extra attack bars (double and triple wielding).
If I only use fists, how many weapons will my char use?
Is there a value of martial arts skill where I start to multi-wield the martial arts item uses as weapon?
Or is attacks with fists limited to just 1 weapon/attack bar?

Related, if I start using weapons with "combined martial arts attack", will these attacks replace those with regular fists?
For example, if I dual wield shen sticks, will this remove any attacks made with fists?

In my mind, it works like this:
Only fists = 1 attack bar (1x fist)
1x shen = 2 attack bars (1x shen + 1x fist)
2x shen = 3 attack bars (2x shen + 1x fist)

Is this correct?

Not exactly.
Using only fists is like 2 weapons (2 fists)
1 weapon + fist is like 2 weapons
2 weapons + having enough MA skill for combined MA attack gives you 3 attacks.

I started a new MA and I only saw one attack bar.
I assumed two bars if I was using two fists.

There is perhaps a MA skill value where I start using both fist (get two attack bars)?

The OP is right, MA fists count as single weapon with 30 attacks per minute.
With one weapon you'll get 40 attacks/min = dualwield and then MA speciality triple wield (fists + left/right hand weapon) = 45 attacks/min.
Equipped weapon must have "combined MA skill" to enable fists use. Without that it works just like any weapon and fists aren't used!