This mod from smcn allows you and a leveling partner to "compress" your inventories via trade.
Inventory efficiency for leveling duo's
Note: v2.0 is not compatible with previous versions of BagPress
Got a partner while leveling and tend to run out of bag slots? BagPress can help! If both players have the mod installed, you can "compress" your inventories via trade.
Usage: /bp, /bagpress, FuBar plugin
When you click the FuBar BagPress button with someone targetted the mod will scan both player's bags for inefficiencies (non-full stacks of stackable items), initiate a trade, and automatically insert items based on what it finds*. You can also add a filter for any items you don't want to trade, and also make a list of items you always want to trade.
Note: The mod must be installed by both players in order to function.
* Please note that for security reasons the trade must be completed by the players, and only items that are meant to potentially be traded will be communicated to other players in a silent manner.