Hi, Anik,
Adding with resistors is definitely simpler, with no worry about instability due to too much capacitance on the summing nodes. This can also be tested with eval. board connections, if cable-length did not post any bandwidth issue. Each amp need to give out twice the signal voltage, so there may be a linearity penalty. I would suggest testing the solution with eva. boards or small test boards first if possible.
Summing at the inverting nodes can only be done with the ADA4930-2, requiring a pcb layout, and the capacitance at the summing nodes need to be minimized.
Since the AD9122 is a current output DAC, it is possible to sum it's outputs through resistors to the amp's, but the common-mode level needs to be shifted through pull-up resistors(9122 output compliance is +/-1V at each output, while 9643 wants to see 0.9V +/- 0.45V). The signal levels will be attenuated like the Amp outputs.
Benjamin
Benjamin