It looks like you are adding a small amount of EXP ( ~50ish) if you combine the 2 small ones first, but the amount of gold you will be putting out per item to upgrade is fairly significant. 3,600 (360*10) + another 1000 (4,600 total) for the last combine as opposed to just 1000 if you upgrade just 10. I find that most items you can nearly hit level 20 with just adding 10 of the smallest items, so the extra EXP you are gaining might not be enough to get more than an extra level at the end of it all (assuming you are levelling an item with 30 max LVL). You might be better off upgrading two items with 10 each instead of pumping 20 items into upgrading one. (Just my opinion....) It will certainly save you some gold which you will definitely need to Evolve your girls....

Give it a check see for yourself... I try to make sure that each girl has at least one of the items then I wait until I have 6 extra or so and upgrade the strongest one I have that hasn't been upgraded yet.

Either way, good luck to you!