Theres alot more variables in play than just the size of the cones.
But for reference one 15 has about the same size amount of cone area as two tens.
Also what kind of difference are you looking for?
Of course going from two entry level tens to some higher end subs there will be alot of differences.
Be a little more more specific
I'm borrowing the 10's from my truck while I search for the right sub for the car. I am using a 500/1 for the sub and really am wondering, in the spirit of fundamentals, what the difference is between maybe 2 300w RE SR 10's and a single 600w SR SE 10, 12, or 15. Between different subs I'm finding arguments for days but I'm really looking for stripped down, variable free answer. I have many brands in mind but this one is about the basics.
Two tens *can* more more air than a single twelve, but how much power you actually give the sub is a variable, as well as the sub design itself - what kind of X-Max it has (how much air it can move, etc.).
If the subs are identical brand and design (except for the diameter) the pair of tens will move more air and play louder than the single 12 (unless you cut the power in half).