This is more out of interest for others than for personal use, as my Soulstone alt is on the same account as my main (I just log onto my alt and trade Soulstones with someone else rather than SSing myself).
But, as you can see below, these are two directly contradicting messages regarding dualboxing a SS alt:
We've made another update to the Multiboxing rules in the Terms of Use to continue to help clarify it.
You may only multibox a soul stone alt if the entire multiboxing process is performed within a major city. If you're logging into one at a time and using another person to trade between your own characters then it's not considered multiboxing as you're only logged into a maximum of one character at a time.