Best bet for that would be Spanish/French victory in the War of Spanish Succession. Both countries would end up under Burbons, continent would be politically changed forever and you would probably see long term alliances that were deemed impossible until that moment aimed exclusively at containing the new super state.
Both countries did end up under the Bourbons and the result was 20 years of improbable alliances arising from Philip V's ambition to regain Spain's Italian possessions and the threat that he would obtain the French throne. Not only was there nothing like a Franco-Spanish union, relations between the 2 countries were as bad as they had been when they were ruled by different dynasties.
The best bet would be for Louis XV to die before he has a chance to produce a male heir, and in OTL it took 14 years between his assumption of the throne (at the age of 5) and the birth of his first son. The problem is that this would result in a civil war between the supporters of Philip V of Spain and those of Philip of Orleans, which is likely to spiral into a European war if it looks as if the former is winning.
An alternate best bet would be for the War of the Spanish Succession to end with an early Bourbon victory, as in
this unfinished TL of mine that I'm shamelessly plugging, and then for a personal union to happen later on, though the conditions for one are not very good given the unlikely course of events that in OTL made a 5-year-old boy King of France in 1715.
Scenario 1 (Louis XIV dies) sets up just the right conditions for Philip V to claim the French throne, but it's unlikely that he can resist foreign opposition to his claim. Scenario 2 (Louis XV wins) makes the Bourbons strong enough to resist foreign opposition to a personal union, but you'd have to walk over a lot of dead Bourbons bodies to bring about that personal union to begin with.
You'd need brilliant leaders in nearly every single generation. Something damn hard to keep up even for just 2 generations, but you know Prussia managed for 2 generations, and after that the weight of Nationalism and Germany managed to turn the area into a truly defined and somewhat unified nation.
No you don't. England and Scotland, Poland and Lithuania, Brandenburg and Prussia, Denmark and Norway, hell, even Castile and Aragon. It is nowhere written that a Franco-Spanish union would break apart just like that.
You know I read a certain written work once that speculated that at the birth at the first mega country more would inevitably follow, and this did lead towards a trend of fewer nations total, and those that did exist were somewhat larger.
It's a personal union, it's not a trend or something you can reliably plan ahead for.
If this occurs expect Italy to either unify earlier than OTL as a result of more pressure on being larger and more unified or much later than OTL as each of the new Super Nations takes their slice of Italy for their own already larger than OTL Nation.
You've no idea what you're talking about.