I'm studying turbojets, turbofans, and turboprops. I can see that, for the latter engine configuration, a gearbox is necessary so that the propeller rotates at a lower velocity than the main compressor. Here is an image:

[![Turboprop](https://i.stack.imgur.com/5PvhA.png)](https://i.stack.imgur.com/5PvhA.png)

For the experts in the aerospace field, my question might be regarded as a bit "odd", but here I go: *is there any reason why we can't use a second turbine to move the propeller with coupled shafts, instead of relying on a gearbox?* The idea would be to use a low pressure turbine that moved the propeller.

I think this might be unfeasible, because the added weight of a second turbine would be higher than the weight of the gearbox itself, but I don't know if there are additional considerations to this.

But on the positive side, we would have the following (and please, correct me if I'm wrong):

* Less maintenance work.
* Propeller rotation shaft aligned with the main engine core (does this *even* represent an aerodynamic advantage?).

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**EDIT 1**

The idea would be using two turbines. A high-pressure turbine (HP) that moved the compressor, and a low-pressure (LP) turbine that moved the turboprop. My idea was taken from the turbofan engine config.

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**EDIT 2**

As the user ***[mins](https://aviation.stackexchange.com/users/3201/mins)*** pointed out, we would need a very slow turbine or even a gearbox. But we could design the LP turbine with few stages (say 1, 2 or 3) so that its rotational velocity would be low enough as to move the propeller without having aerodynamic losses in the tips of the prop. And as this same user pointed out, there already exists such a configuration (see [here](http://theaviationist.com/2015/02/15/bear-contra-rotating-blades/)). The question then is, why isn't this configuration preferred over the "conventional" one? (i.e. the one that uses a gearbox).