I don't recall the Germans ever having seriously considered such a thing.
Simply wrong. Schlieffen himself came - in a way - back - to ideas of Moltke the elder in 1903 (
Generalstabsreise Ost 1903 General staf journey, thats how they called and made kind of manovering tests on plans).
Their original Schlieffen Plan called for 90% of the land army to crash through France and the Low Countries, hopefully knocking them out of the war in time for a pivot east to focus on Russia. In the intervening time those forces in Prussia and eastern Germany would have assumed a defensive stance in the hopes that France could be defeated before Russia totally mobilized.
just to remind : the "schlieffen-plan" was originally a paper on overall strategic possibilities, that ofc included the idea of first beating Russia and then France.
What most people call the "Schlieffen-Plan" was (just) the one variant that came into practice and was favored by Moltke the younger.
Unfortunatly, the materials of the "East-first"-varaint of Moltke the Elder, his successopr, Waldersee and was still considered up tp 1913, when Moltke the Younger decided for the "West-variant", that have been archived were after that decision and due to later courses of war (WW II) scrapped and lost.
There is just almost nothing of these plans and papers left.
However, what is known of these plans, mainly from the Waldersee-time, called for an approximatly 1/3 of troop in the east, taking on Russia together with A-H and 2/3 in the west on defense against France.
Schlieffen put up a variant of 1/2 in the east and 1/2 in west in about 1903/1904. The "last" variant of the Schlieffen-plan (stratetic) include an "Aufmarsch West I", an "Aufmarsch West II", an "Aufmarsch East I" and an "Aufmarsch East II" dealing with different possible political alignments.
But as said earlier in 1913 Moltke the younger scrapped all east-plan for leaving only the west-variants and tried to "optimize" them.
(main reference : the
german wiki-site on the Schlieffen-plan)