I have a few Phalaenopsis that I purchased with or that subsequently developed side growths. Something ended up being wrong with the parent plant in 5 out of 6. I’d be reluctant to separate unless there’s no workable way to pot the side growth and parent as is. If the parent plant doesn’t seem to be thriving, whatever is going on may have triggered the side growth.
If i recall, within the first year that I acquired his phal it did get stressed out from something. For a while the new leaves were smaller than the previous leaves which is an indicator that the plant isn't at the top of its game. At some point that basal growth emerged. Somehow the mother plant recovered and has been doing well and reliably flowering almost every year
If the basal growth has some roots in the substrate, it’s possible that the basal growth is supporting the parent plant. Stronger together If you want to separate, you can try but it carries the risk of weakening both plants and depending on how reliant one is on the other, the weaker plant may or may not recover.
One thing is ------ out in the wild, generally no person will be there to divide the orchid in situations like this. So if the orchid were to split in half ------- then it would be down to some sort of input from nature.
On the other hand - as the outgrowth plant is so large and the roots are so well developed, there would be more than a good chance if it were divided manually. Drying of the wound before potting ----- mentioned by Ray already - is a good call. Or while I haven't tried it before - graft/pruning sealant or plant sealant 'paint' ------ could be workable too --- also requiring drying/curing before potting.
I'd ask the question, "Why even consider dividing?" I don't see any upside at all... the downside is the likelihood that one or both parts would die. Just because maybe one can do something doesn't mean one should. The original and new growths together make for a stronger plant - with the very good chance of getting spikes from both growths, making a much more attractive and dramatic display. When it gets too big for the growing area, then division makes sense. But that's not where it is at this point.
In this case - the OP's reason was along the lines of the parent plant being burdened. But the parent plant isn't being burdened at all. So if it's ok to not divide ----- then might as well just allow it to keep growing like that.
On the other hand - if it had been some other reason such as - for making a back-up, or want to give that portion to somebody else to grow ------ then it should be no problem. Naturally there are risks ------ so it would be necessary to plan ahead before dividing --- and know what to do to really cut down on chances of issues.