not normally "recommended" relates to one of the instances being prod and that is presumably because of the risk of getting it messed up and down time while restoring. With test and training the effect of the risk is lower. That is not limited to MS SQL, but applies to Oracle as weel.
If you read the installation guide it says something like that you can name the database whatever you want, but the database user name has to be "niku"
Read it.
If you go ahead and have two instances on the same db server are you going to have one or two app servers? You can have two instances on one app server if you differentiate the with all different ports.