It's possible to do that but as the aggregates are already created you'd have to destroy the root aggregate (effectively the controller) on B and rebuild it with 12 disks, then give the other 12 to controller a and create a new 12 disk x 2TB aggregate. if you add them to aggregate 0 with the 830s it will 'rightsize' the 2TBs down to 830.
You won't gain that much by doing this and it's normal to have mixed systems. What is it you're trying to achieve?
If it's a brand new shelf you can expand controller b's aggregate by 12 disks and add a new 12 disk aggregate to controller a. This is supported and probably your best bet. Might also be worth poking your reseller as some of the new 2240's have better capacity and there are usually upgrade offers on at this time of year.
It's possible to add the disks to the aggregate on a either a new RG or add to an existing (depending on the amounts in there already). The only caveat would be that if you add the disks to the Aggregate with smaller drives already, they will be downsized to match. In this case you will get a better return on the size by creating a new aggregate on that controller. Any volumes that are short on space can be move between aggregates with the vol move command.
It's a bit easier to see what is going on if you use the OnCommand System Manager to do the disk addition. This is a free download.