You can do seven: Connect the trigger out of the MSO to one of the input channels of the 1054Z then trigger off that.
nb. You may get more accurate timing if you connect the DS1054Z to the same signal as the MSO and set up the same trigger condition.
That only works at slow time/div settings where the added delay and jitter of the "trigger" output, which is not a trigger output at least on those DSOs, is small.
In the more general case, the only way is to connect both instruments to the same trigger source and use the same trigger settings. In the past, a "trigger recognizer" might have been used as the trigger source for multiple instruments, which might have been as simple as a word recognizer, or analog oscilloscope or universal counter with a gate output.