Each case is a case.
It is possible to use both and make them communicate through webservices or something similar. The intention of the webservice, that is not always the best solution, is precisely technologies can communicate without understanding the details of the communication and functioning of the other party. It is used to standardize access to data from one application by another application.
Of course I would try to make a homogeneous application whenever possible, and it seems to me that this case is not only possible, it is desirable.
I doubt it’s worth the trouble of creating webservices to take advantage of a component that helps you with some task. Something that can probably be done easily also in C# and that should have something ready to use with this language. But if you need webservices for other reasons, there already begins to have advantages.
That is, understand why you are choosing each one, but do not separate the application in two just because one language has better components for one thing and the other has better components for another.
There is nothing that Delphi has for Windows applications that C# doesn’t have the same or similar improved. Go to C#.
– Caffé
How would these components for a commercial automation application that Delphi would be better than C#?
– Arthur Menezes
Acbr probably, but I think q they have a dll to be consumed by C#
– Caputo