So if you want to play with one another and don't want to start from scratch here is a working mod for online games:
Borderlands 2 Save Editor (Revision 155)

Download.
Known problems:
Setting a character's level
In the 'General' tab, type in the level you want the character to be in the 'Experience Level' field, and then click on the 'Sync' button of the 'Experience Points' field to automatically compute the appropriate Experience Points for that level.
Unlock all 4 weapon equip slots

Unlock True Vault Hunter Mode
Go to the 'Raw' tab and set PlaythroughsCompleted to 1.

Currently I have a legit level 5 sniper dude. But when the 25's come out to play I also have a level 25.
Borderlands 2 Save Editor (Revision 155)
- Bug fixes and updates to game info (“Psycho Pack”, “Loot Chest 2″, “Poker Night at the Inventory 2″ DLCs) for the current Borderlands 2 patch (1.5.0).

Download.
Known problems:
- Eridium in-editor will show up to 99, the rest is stored in a new value that’s not handled yet.
- Expansion playthrough number and playthrough completions are also stored in a new value that’s not handled.
Setting a character's level
In the 'General' tab, type in the level you want the character to be in the 'Experience Level' field, and then click on the 'Sync' button of the 'Experience Points' field to automatically compute the appropriate Experience Points for that level.

Unlock all 4 weapon equip slots
- Go to the 'Bank' tab, then click on the 'New Item' button.
- In Type, select GD_StorageDeckUpgrade.A_Item.INV_SDU_WeaponEquipSlot (near the bottom of the list)
- In Balance, select: GD_ItemGrades.StorageDeckUpgrades.ItemGrade_SDU_WeaponEquipSlot
- In Manufacturer, select: GD_Manufacturers.Manufacturers.Stock
- Keep the rest of the fields untouched.
- Repeat the above to add another weapon equip slot SDU (Storage Deck Upgrade) or right-click the created item and then select 'Duplicate', to add a total of two SDUs (to unlock the remaining locked two weapon equip slots)

Unlock True Vault Hunter Mode
Go to the 'Raw' tab and set PlaythroughsCompleted to 1.

Currently I have a legit level 5 sniper dude. But when the 25's come out to play I also have a level 25.