It sounds dangerous as heck. Your local electrician doesn't sound like any electrician I'd want working for me
How many breaker spaces are in your panel. And what is the model number on the panel? The number of allowed breakers will be part of the manufacture data that's on the inside sticker of your panel. It may say 20-30 or something like 32-42. This will mean how many, if any, tandem breakers are allowed in your panel. Some have spaces for these at the bottom, the lower spaces on the buss bar. If they're not allowed in this panel then it's dangerous to even think about putting them in there. Then you would need to install a subpanel for any new circuits.