Can I convert a single sink to a double sink without major plumbing repairs?

Can I convert a single sink to a double sink without major plumbing repairs?
Since you mention bathrooms I'm assuming you wish to swap a single lavatory basin vanity for one with a double bowl. Yes?

N o problem! Simply connect the two bowls together with a continuous waste, (see image, I have shown a center outlet and one with a end outlet.) and use the same trap for both. Good luck and thank you for rating my answer. Tom

The answer and picture that speedball gave you would work if your bowls are going to be close together but if you have drawers or a leg space between them I am not sure it would work. You might have to do some changing to the drain line and to the water supplying the faucet for the second bowl.