Elina, I fear you have a fairly fundamental misunderstanding of how Marketo smart campaigns work.
Without getting bogged down in the details (I think you should start with the docs and a lot of self-learning/self-testing) a trigger SC runs on the Marketo server to allow you to automate/schedule a huge range of follow-up tasks in the Flow tab. It doesn't run in the web browser, so there's no way to set a Clicks Link in Email activity to do anything but, well, take the person to the clicked CTA.
(Also, for completeness -- as well as confusion I'm sure -- Clicks Email doesn't even have to open a web browser proper. It's an HTTP activity but not necessarily a browser activity. Though we certainly want it to open a web browser to the CTA URL it doesn't have to in order to register as activity. For related reasons, you can't even trust Clicks Email to have been done by a human being, it can be done by automated mail scanners.)
Again, to try to avoid dealing with the remaining elephants in the room... if you have 2 Landing Pages, they have unique URLs. So only put one of the URLs in the email. Not sure what kind of setup you've got where that wouldn't be the simplest, most appropriate approach.
Need more context to fully understand what you are trying to accomplish.
What is determining who gets what landing page? I know you say "Source" but if you're pointing to a landing page via an email, the "source" always going to be the email. If you're looking to point people to a specific landing page based on a particular demographic, filmographie, or behavioural aspect (e.g. CXOs get X landing page, and VPs and below get Y), then this can be achieved through the use of Segmentations and Dynamic Content. Option one would be to create two landing pages, then edit the email so it's dynamic, and change the link based on the segment you are trying to target. Option two would be to make the landing page dynamic, so that when the lead visits it via the email, they'll be served the content that you want to show them based on their segment.