Maybe next time just ask if someone can help you
At some point it was decided that there won't be any trading between alts and mains.
People could still transfer their gold and heirlooms over to other chars by trading them to a friend, then deleting their main and setting the next alt to main if the system wasn't working as it is.
From time to time this gets reconsidered but so far decided against and if there ever is a system to retrieve your heirlooms from your alts and stuff, the main/alt system will most likely stay as it is anyway because of Strategus and/or lazyness
Point is, everything is working as intended and it takes about 5 seconds to fix this for you.
Normally we do not reset char roles but the above reason does not apply for you as both your chars are Gen1 and one is STF i will just reset their roles so you can select a new main.
The other one will be tagged as alt and will stay like that forever.