I have a problem with my 2 DVD burners LG H55N and Nero

I have a problem with my 2 DVD burners LG H55N and Nero
Burning a DVD image on both burners using Nero option “use multiple recorders”, Nero will set up the speed on 4x and I can’t change it. The discs are Verbatim DL at 2.4x.
The buffer will oscillate 4% and 97% from one burner to the second one. The time to burn the DVDs will be 1h 30 min. if I burn only one DVD will be ready in 28-30 min at 4x …again I can’t change the speed. Before introducing the DVD in the burner I have the option to change the speed. I have 2 computers and this is happening only to one of them. Second computer will burn 2 DVD on 46 min. (burners LG H10N and Pioneer 111D)
please help

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