While there is no laws against dual employment, there are generally contractual terms that prohibits dual employment, thus if a company finds out about it, action can be taken against you.

There is a conflict of interest, as you are not honouring the labour agreement you signed with both companies. Furthermore, you are using resources from them to benefit the other, data, information, training, etc etc.

Most companies will not allow moonlighting unless it's explicitly approved.

You agreed to be paid X amount in exchange for X contribution, such as time, days, results etc.. so contract wise dah not okay.

If you go tell them or they find out, they can terminate you depending on your contract. Law wise, nothing stopping you, work wise, cannot unless agreed in advance.

Just keep your mouth shut and continue but be careful la.