Isn't one way of envisaging this:

Make a table showing the answer you get.

if the___________If you picked
liar is________Jon______the other
Jon___________?__________?
the other______?__________?

To satisfy both questions you must one cell must be different from the other three. But a single question can only distinguish between the rows OR the columns.

Thus 'Are you Jon?' gives

if the___________If you picked
liar is________Jon______the other
Jon___________No__________No
the other_____Yes_________Yes

'Does Jon lie?' gives

if the___________If you picked
liar is________Jon______the other
Jon___________No__________Yes
the other______No__________Yes

and so on.

If you ask both questions in succession then you get four combinations.
No,no - you picked Jon and Jon is the liar
No,yes - you did not pick Jon, and Jon is the liar
Yes,no - you picked Jon, but Jon is not the liar
Yes,yes - you did not pick Jon and Jon is not the liar

Or, as a table

if the_____________If you picked
liar is__________Jon________the other
Jon___________No,No_______No,Yes
the other______Yes,No_______No,No

If you are interested in who is Jon, that's the question.
However, if you are concerned with whether Jon is a liar, you should look for another question.

>Yeah only took me a whole week

You did eat, did not you?

That will only tell you if the person you are addressing is Jon or not, but not the identity of the liar.

Any question with a yes/no answer won't work, because there are only two possible answers -- but we have four combinations to work with. Calling the twins A and B, the combinations are:

A is Jon, A is the liar
A is Jon, B is the liar
B is Jon, A is the liar
B is Jon, B is the liar

Whatever question it is, it has to be open-ended.

Possible answers
Yes
Yes, but my name isn't Jon
No
No and my name isn't Jon

How about the answer "My name isn't Jon"

Have a Good Day
KJ Ramsey

You ask the twin a single question to derive 2 elements, identity and disposition towards the truth. No question can "force" them to address both elements.

It's not actually a sentence at all. Nor, formally, is it a question. If it were one sentence, that wouldn't mean it was one question: it's quite easy to formulate several questions in one sentence.

Moreover it doesn't work anyway. If you happen to ask the liar and he says 'No, and my name is Fred' - which is likely - where are you? You know he's the liar, and his name therefore is not Fred, but that doesn't prove it is or isn't Jon.