Heh, I found out how to solve this little issue:
With Terraria 1.0.6.1 downloaded, rename the entire game folder of the game to something else, like "Terraria 1.0.6.1".
After that, leave Beta and click Update game to download Terraria 1.4.4.9.
On Steam, click Add Game, then Add Non Steam Game, and select Terraria 1.0.6.1 executable.
There, now you can play Terraria 1.0.6.1, Terraria 1.4.4.9 and tModLoader.
Triple profit.