You are free to create as many accounts as you like and you are welcome to trade between your accounts just as you would with any other legitimate player.
However, if any of your accounts obtained wealth by rule breaking activity for example macro use (botting) or real world trading (buying gold or items from 3rd parties) then all your accounts will be banned.
Similarly, multi-logged accounts must not interact with each other to exploit gameplay mechanics, such as attempting to rig the result of a minigame.
Trading wealth or items between your own accounts is done entirely at your own risk. Unconventional trade methods like drop-, death- or trust-trades are particularly risky and we strongly advise against them.
You may log into multiple accounts across all versions of RuneScape at the same time. However, any account you are using must not be involved in any rule breaking or exploitative activity. Any penalties or sanction for such behaviour will be applied to all of your accounts.
Why we have this rule
We have this rule as part of our procedures to monitor and deal with people who choose to real world trade, gold farm and use macros. People who move wealth between accounts they own and have gained that wealth unfairly or by breaking other game rules can expect to be banned.
It's allowed to trade from your alts since you get them the rewards legit.