If the business is scalable, you can try starting on a small scale. A colleague of mine did that, and last I heard, their business is now booming. Both spouses were salaried employees at the time, and one spouse continued to be a salaried employee. So it's possible but, from my observations, at least, it's the exception rather than the norm.
Running your own business requires focus. Nothing moves properly unless someone orchestrates the business - and as a startup, that 'someone' has to be the founder(s) because that's where the vision / direction comes from.
Now, that doesn't mean you have to give up your day job as (say) a software architect to run a consulting business, but sooner or later, someone will want to meet you during office hours or a problem will crop up and you will need to be on hand. This will impact your day job.
It's possible in theory to continue working as an employee while starting your own company, and we can even point to people who have done it, but something will have to give. If you use your after-work time, you're giving up the weekend get-togethers or family time. If you get someone else to do it, you give up a measure of control - and it'll almost certainly cost more.
Also, the risk appetite of a salaried worker and that of an entrepreneur is normally quite different. Regardless of how good business gets, there can always be slow periods ... but employees still get paid.
Pulling all this together - the entrepreneur has blue-sky potential, but the risk is commensurate.
The way you phrase your question suggests that you value the stability of salaried life and you're wary of jeopardising that. Treat the business prospect like any other investment: quantify how much capacity you are willing to provide in time and money, then see if that's enough to get things moving, perhaps alongside others who likewise are willing to commit quantified amounts of their own time and their own money.