the most important thing first: your idea of coupling two outputs directly WILL VERY LIKELY DAMAGE THE BOARD. This being said, let me reply to your original request:
Unfortunately, you named the PXI chassis and the connector block type, but not the type of the actual PXI card that you are using to output the signals. Please unscrew the TB-2705 connector to read the PXI card's four-digit type code and post it. Additionally, you should clarify which NI driver you are using in MatLab. Did you mean the Traditional NI-DAQ driver?
Generally speaking, the solution for your problem will be to create an output task that uses "streaming". This means that you will not download a complete, arbitrary waveform to the DAC card's memory which is then output in one go, but that you continuously provide the data "bits" for the next couple of milliseconds. This way, you can programmatically decide when to change the signal's amplitude, frequency, form, ... without having to stop the output itself.