I will use now three SC16IS740. They should work the same as the 750. The 752 with two UART's do not work and I do not see the mistake in my code.
But the other way, if I use three 740 or two 752, the size of the PCB I will need is nearly the same. The coding with the 740 or 750 IC's are better, because I can give each IC a specific name for the UART. The 752 needs channel A and channel B what is not that easy to write the code and needs more to define the UART's.
I think it is really not bad to use the single UART IC's 740 or 750. For me it is the better choice now.
But anyway, if someone could get the 752 working and find the mistake in my code, please write me. I am very interested in this why it is not working... 