I dont know where to post it so there is it.
I know 3 ways to stack skills. (It will only work on Interlude old database server i think)
1: Make any human mystic character, start all the quests to become both the 2 kind of mages.
Do the quests but do not complete them, so you have the quest items on ur inventory.
Take 1 way, get lvl 80, go back to the high priest in gludin and ask for a first class change.
Some years ago it was working in 3 servers, and in another it was high priest of gludio.
With this old trick (which rarely work), u will get back to lvl 20 and have to lvl up again.
2: Take a first subclass (exemple warlord), then a seccond subclass (exemple prophet).
Take ur prophet subclass, then go to the teacher but do not learn the skills yet. The skill learning window should stay open. Go change of subclass to warlord. Go back near the teacher and try to learn a skill. If it does not work you can go near the other teachers until u can do it.
3: U need a server which the subclass quest is automatic. take a subclass, go to the teacher and open the window to learn skills. go to cancel ur subclass when this window is still open. go back near the teacher and click over a picture to learn a skill.
Sometimes all these tricks does not work, so you have to be smart and do both 3 trick in one.
In all servers i have been i could stack skills from different class, so you should be able to or find another way too.