Whilst I do agree that vit twins are mandatory for (not-so) cheap and easy farming, they are more of a necessity only later on (read: levels 70, 80+).

With some good pluion/edwin and a decent (AD) staff with leech, having int twins alongside your psy ones will be some nice extra damage output. This will allow you some time to do the twin dungeon (minor/intermediate) to farm some (vit) twins. Hopefully with more luck than me, as I needed 11 vit twins to have a working set... it will also make for faster leveling.