I will start by making an assumption that what the OP is asking here is 'can I hold down two full time salaried positions at the same time'.
So this gets real tricky. When you are a salaried employee, especially an exempt salaried employee, it is assumed by your employer that you are giving ALL your time, attention, and talents to them.
Lets say it's even a bit more complicated and you are in a creative field. Now you create this amazing logo for company A and company B finds out you made this logo for company A, they may have a legal right to that logo now. This gets into a lot of strange employment law, but really centers around how in the US anyway a salaried employee is essentially working FULL TIME, as in 24hours a day, for the company. Some companies will go so far as to put this in your employment contract even and it can be pretty onerous in some cases I have seen about limiting what you can and cannot do.
Now if your situation is a bit more mundane and you are cog in the wheel of machinery working 8 to 5 and then working 6 to 2 in another machine you might be OK. You could in theory, even with significant contractual limits say work a 'day' job as a creative (engineer designer etc.) and then work as a barista, bar tender, ticket taker, whatever, on the nights and weekends. It really is very specific to your situation.