To synchronize clock of several UmTRX you have to leave one of them in master mode,
but all others must be switched to slave mode.
Master/slave control switch for 26MHz clock is S3(MHz).
I/O connector for 26 MHz clock is X7.
In case of master mode X7 becomes output and input in case of slave mode.
To set master mode turn switch S3 to the downward and to upward for the slave mode.
Please find text on the PCB board to be sure.
BTW, if you would like to use external clock source then all of UmTRX's must be in slave mode,
i.e. their clock connectors must be inputs.