Thanks to Artcp, who has pointed out a possible way of cheating/abuse in the game using singleplayer mode:
When playing solo and loading multiple characters, the gold/ancient crystals that is loaded last is the one that counts. Being so, a player can have a single character with 8 crystals, start a game with all characters and save the 8 crystals for all of them. Furthermore, the place can use the 8 crystals to buy some epic items, save it, then use the already mentioned load/save to get the 8 crystals back and keep the equipment.
This issue is kinda tricky.
I have decided against the solution to use different types of save codes for singleplayer and multiplayer (because SP heroes should be used in MP).
Thus, since I haven't found any other decent solution for this issue, as a consequence, I have disabled the option to choose more than one hero in singleplayer.