Hi Ross,
I would recommend you to have a look at this DJI M210/M210(RTK) Dual Gimbal Support community thread to get the latest news about Dual Gimbal Support.
However, Pix4Dcapture supports DJI’s single downward-facing gimbal so you can consider flying two separate missions with your X4S and X5S cameras.
For an optimized reconstruction, we advise that you keep a constant ground sampling distance between overlapping images. Therefore, we do not recommend that you have several GSDs for one project, which would be the case for the mission you described because the X4S and X5S have different sensors and focal lengths. It may affect the reconstruction or even fail.
I understand the theory behind your proposal, considering that the GSD you will capture with your X5S will be much more detailed than the X4S’ GSD. However, we must also consider the amount of image overlap that you will capture because I anticipate that Pix4D’s image processing engine will not calibrate the X5S images due to the significant difference in GSD between the X4S and X5S, which may yield significantly different distributions of Automatic Keypoints in overlapping X4S and X5S images.
Nevertheless, please let us know the outcome of your project if you fly a mission that matches your original description with another third-party application.
best,