and this ID kept changing.

Of course the ID is always changing, that is the correct behaviour and this is what you need. As explained before:

1. Whenever someone opens a survey link on Limesurvey, the system creates an ID for this respondent. This is the SavedID. This ID is unique for each respondent to a survey and as you can see it is counting up. This ID exists no matter if someone completes the survey or not.

2. If you pass this SavedID from your first survey to the second survey and save it in a hidden question, you have the possibility to match survey 1 responses with survey 2 responses. It will not prevent people from filling the surveys twice, but it will show you if someone is trying to cheat in survey 2 by using the link to the second survey more than once to increase chances (if there is the same ID more than once saved in the hidden question of survey 2) or if someone is trying to cheat in another way (e.g. if there is an ID in the hidden question 2 that does not exist as saved ID in the first survey). Maybe, you might want to add something else to that SAVEDID to make it no so easy to guess and cheat.

E.g. you take a hidden equation question and create a more complex semi random number. E.g. 3 random letters, than the SAVEDID and than 2 random letters or something like that. Save this in the hidden equation and then pass this ID on to the second survey in a hidden question.

Also not sure what your screenshot is showing. Is this from survey 1, is it from survey 2, what do you want us to see?

The first column in your screenshot is the Savedid from the survey you are showing us (12, 13, 14, 15, etc). Now, each of the two surveys has their own Savedid of course. But for your problem the relevant Savedid is the one from survey 1, you pass it on to survey two and write it into a (later hidden) text question so that later you can be able to compare and see if someone is cheating. You can't prevent cheating, you can only identify cheating easier afterwards.

E.g. if your system automatically provides a voucher to respondents in the second survey, all this effort is in vain, because they will already have the voucher code. You will only know afterwards if they cheated.

Thank you very much for such deliberate answer!
it is Survey 2 on screenshot.

Ok, but where did you save the Savedid from survey 1? Can't see it in the screenshot. The "ID" you can see, is the savedid from survey 2. You don't need this one. You need to save the ID that you are passing from survey 1 to survey 2 somewhere in survey 2.

That's the Survey 1. Is it correct? You see here that responses in survey 1 ends on 11, while in Survey 2 - on 15.
I didn't do anything except changing the endlink. Should I do anything else?