If I’m reading the supply rules correctly:
12.1 Tracing Supply Lines
12.1a A Supply Line Trace is a path free
of enemy units and EZOCs between each
unit and the side’s supply source.
So I would have to make sure I leave units behind that connect that lead to the supply source of each respective German/Russian to be in supply?
If I understand your question correctly, I would ask in return why would you think you need to leave units behind?
In the simplicity that is SCS, supply trace is simply that the supply trace path from your friendly unit back to the game's specific supply source be free of enemy units and/or EZOCs and that path may be any length. Friendly units can negate EZOCs if need be, but hexes traced through from the friendly unit to the supply source do not need to be occupied by a friendly unit or friendly ZOC.
In SCS check the game specific rules for any differences between the SCS general supply trace rule and the game specific supply trace. For example in The Mighty Endeavor, Allied units must be able to trace no more than 5 MPs from the friendly unit to a nation specific HQ (e.g. US unit to US HQ, British unit to a British HQ etc.) which in turn traces to a supply source. For the western Allied forces the supply source would be a functional Port.