Well the scientific argument is that the two cabs will have different responses and cancel each other out, plus 15s are not inherently bassier then 10s. ...
I'd like to agree with the second part of this but the first, while scientifically true, still may yield a great tone. I've played an Eden D210T and D115T speaker set up for years. I can tell you that in this set up the 210 is actually deeper and cleaner. While the 115 has a nice mid hump with some growl that the 210 does not get. The 115 gets the rock Pbass tone while the 210 has a wider (slightly scooped) tone that's more "modern". So I have these options as seperates or together I get a great combination of the two that is loud and versitile.