If you want them stable...cut of each of the lower branch segments and the main part, let them dry and callous for a couple of weeks, and plant again. Just watch out for the sap, its toxic.
This EUPHORBIA lactea(not a cactus, just looks and works similar to it) habit is shrub to tree like, often with dense foliage
Judging from your reply.. you want it outside but cant plant it full time in the ground or its cold or ...? it was either cold or too hot(the latter of which can be mitigated)...
I suggest similar methods if you want it to grow outside..
