It's not possible to work two full time jobs, at any point you are cheating the other hence why they are secret
Depending on the employer you can work two different jobs so long as it is declared.
Especially works for contractors and I know a couple who do this. Balances out the peaks and trough, retains skilled workers and also allows you to work doing different things so can benefit both.
The only issue is if there is a conflict for their time, but typically there is some agreement for how this is managed from the outset.