Mixing multiple sources into a single destination can create problems when the output impedance of modules is not matched. One of the benefits of using modules from the same maker is that they typically have matched output impedances (typically 1kohm in eurorack modules). In the case of matched output impedances signals will mix equally, though also be attenuated according to how many are attached - eg. mixing 2 will half their volume, 3 will divide by 3.
Working within isms, particularly if you’re using primarily monome & mannequins modules, will yield largely predictable results when mixing outputs with stackcables.
The inverse connection (eg. sending one sequencer to two oscillators) is the typical stackcable connection, and is the primary purpose of the design. There can be some voltage ‘droop’, but I’ve never found a need for an active mult for 2 or 3 destinations.