I use it to charge two batteries but can I use it to put two batteries in storage mode?

Sounds like it was not designed to put batteries in storage mode. 

Yes, you can use it to put the batteries into storage.

Gonzo,
 I just bought a new balance adapter from EP BUDDY, it came with an important correction recommending to "plug all the power connectorsDeans,EC3,EC5,XT60,etc.)1st, leave them for a couple minutes, then connect the balance leads. By this way the voltage difference between different packs can be minimized, and the possibility of damage to the board can be eliminated if there is a significant voltage difference between batteries."
.                                          Bob

I wish this topic would have come up awhile ago, it might have saved the generic parallel board I bought. I started connecting the balance leads and vaporized the foil trace off the board. I salvaged it by hardwiring the bad traces.
.                                           Bob