Dr Wolf:
Have a look at:
https://github.com/analogdevicesinc/iio-oscilloscope/blob/master/block_diagrams/fmcomms2.svg
There is an output mux before the internal "PA". I don't think you can run both outputs "A" and "B" simultaneously. (You can run Tx1 and Tx2 simultaneously, the phase delay between them is going to depend on how the digital ?& analog filters are set up. Assuming they are the same - it should be within 1/2 of a sample).
Tomas can confirm.
-Robin