With a PoD no earlier than 1856, have a Unified Germany and a France governed by a republican government, be allied militarily(bonus points for economically and even culturally). Extra cookies and more bonus points if their eventual alliance grows into something like the later European Community and includes other great powers like an alt-Italy or the Habsburg Empire, in any form they may take. Maybe an earlier, larger and alternate Latin Monetary Union could help, maybe they could unite against a common threat(Anglo Russian alliance?).