We don't have an external trigger but this can be done in principle. You need two things:
1) configure the sensor as "slave" by setting some registers in it
2) Input external VSYNC/HSYNC signals at the connector. With an extension cable and PCB similar to this you should be able to get the job done: https://www.aliexpress.com/item/32811483374.html?spm=a2g0o.productlist.0.0.41bf5dd70r1xDP&algo_pvid=3a9b4b5f-9c6c-443e-91a4-5356cb20c1e0&algo_expid=3a9b4b5f-9c6c-443e-91a4-5356cb20c1e0-2&btsid=ac009a96-79af-4e85-ab65-c35bcc175f44&ws_ab_test=searchweb0_0,searchweb201602_2,searchweb201603_52
but you should design your own PCB that will send the external VSYNC/HSYNC to the appropriate pins on the camera sensor (see below).
Note that we have not tried this, so you would have to experiment. See registers COM5 and COM10 at
http://www.dragonwake.com/download/arm9-download/OV9650.pdf which deal with slave mode.
From what I see in COM10, looks like you would need to provide both external HSYNC (to repurposed RESET pin) and VSYNC (to repurposed PWDN pin).
We provide an interface to set those registers from userland with no need to hack the kernel. First you issue
setpar camreg 1 # allow users to mess with camera registers
getcamreg REG # get current value of a register
# modify the value
setcamreg REG VAL # set the new value
For a software workaround (each camera will still be on its own clock and the frames will not be synchronized), see