Alternative play for Splendor
I play quite a bit of Splendor, love the game, but I've never run into this problem. Do you each pay attention to the other's gameplay? (that sounds rude, but I promise it's not meant to be!) Do either of you ever sabotage the other? 

For example, if I see that my opponent's purchase power leans toward a particular premium card, I snatch that card off the market, along with a wildcard token. I'll score it later. 

Also, if I notice that a particular gemstone is hard to find, I'll buy those up to slow a leading opponent. 

Maybe you already do these things. 

If not, try paying closer attention to what your opponent's cache could purchase, at any given time, and if it looks like he could spring ahead by purchasing one of the 4 higher level cards, then snatch that card, along with your wildcard token, and shout, "Punked! B!tch!"

That last part is very important.