Hello again and thanks for the help and the hint. I see why it should not be possible to transmit and receive at the same time with software serial.
I solved it in a slightly different way by connecting the HW TX lines and RX lines between the Arduinos to make them receive the same message at the same time. This made it possible for me to eliminate one intermediary. After this I used the AltSoftSerial of SoftwareSerial and let one device send the information and the other look for it on the software pins. It works great now!
Thank you!