I wish to add a garden light on an existing outdoor circuit running between the garage and the house but unfortunately (1) the electrician who wired the house 5 years ago did not run the neutral between the last light and the two way switch in the house (to save on cable?) and (2) I cannot run a new neutral at this stage (it cannot be encased).
An electrician tells me that I need to create a new circuit with a different switch for this new light, which is not what I have in mind. He also tried to use the neutral of a nearby socket and to connect it to the circuit but the fuse tripped (I don't have an electrician background but I don't believe that a same circuit can share two neutrals).
Is there a technical solution to this problem or should I give up on the new light idea? Many thanks.

A bit difficult trying to follow what you have for existing wiring. Sounds like the circuit is fed by the garage, where the neutral begins, and ends at the last light, and the switch wiring continues onto the house, and a neutral would not be needed at the switch.

What is this wiring encased in? Conduit? Direct buried cable?

This should be as simple as installing a two wire cable(plus ground) from the last existing light onto the new light.

Any sketches or photos may help, if I am not correct.

He apparently ran a switch to the light and there would be no neutral. Not sure how the electrician wired you "new" circuit, but if you run a neutral, hot and ground from the same outlet, it should work. It's no different than putting another outlet as a continuation of an existing circuit. What you cannot do is run 1 hot from one circuit and a neutral from another.

Thank you tkrussell for precisely wording the current situation. The cable (without neutral) between the last light and the switch in the house is buried under stones and it would be a big work (and costly) to lift the stones (which are cemented) and to burry the cable.
As ma0641 says, I cannot run 1 hot from the circuit and a neutral from another.

Is there a way to add the light without a neutral (I am not an expert!) or is there any alternative solution to (1) cabling th last light to the switch or to add the light to a new circuit? Many thanks for your help.

I see no other option other than adding a light with a new circuit and switch.

There are some solar lights that have a pretty good output but not as a flood light. Do you have an outside wall outlet nearby? Perhaps you could take off there and use UF cable.