I believe you are more concerned with why a single motor should be running using 2 variable frequency drives (VFD):
The primary requirement will be catering critical application where the shutting down of the motor is not feasible just in the VFD fails so we go for redundancy concept which is possible in certain makes of VFDs.
Load sharing is also a requirement in order to avoid loading of a single VFD and reducing its life, where 2 VFDs of motor FLC is selected but ran on 50% load sharing, so the VFD is not fully loaded at same time if one fails the other can takeover without any cut-short in output performance. 
Almost all high HP variable frequency drives used in application such as drill rig top drives, draw works, mud pumps and marine propulsion use parallel VFDs. The VFDs are fed by a converter in a common DC bus configuration and most cases will be a diode bridge or active front end. Some impedance is typically used on the output of the VFDs. These are referred to as balancing reactors and are used to slow the rate of current rise on the output of the VFDs. This provides some margin of error related to the firing of the IGBTs in the different inverter modules. When one looks at high HP packaged product from companies such as Gozuk or ABB they may look like they have just one VFD but when the enclosure doors are opened, it is easy to see the parallel output VFDs. An advantage of using liquid cooled VFDs is that the current density is much higher and it may become unnecessary to parallel output VFDs.
This functionality can be incorporated into VFD during controller design and programming. Some VFD manufacturers might already have such functionalities programmed. However, I do not feel comfortable to assume that VFD can be run in parallel by default. So, the best answer is to check with the functionality of a particular VFD we are dealing with. Moreover, even if we have VFDs with such functionalities, make sure that they are of compatible rating. I do not want to overload and damage a VFD by paralleling variable frequency drives of different capacities.
You also can parallel two VFDs to increase horsepower. It works very well. I have done this with many medium voltage drives when the client does not want to go with liquid cooled VFD's for an increased horsepower. The only downside is that you require a PLC, however maintenance is much simpler and you don't have any concerns about coolant conductivity, leakage, or particulate matter.
Harmonics will play a role in this case. Design the system accordingly with VFD expert rather trying something adventurous. Lots of factors play their role in selection aspects.
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