How would they ever know if you won't outright tell them yourself (like being a famous person an posting your itinerary on the instagram)? They'll see you've been to Russia, if you use the same passport for both visas, which may be frowned upon right now, but not illegal. But which Russian cities you've been to (and de-facto Mariupol is just that right now) — who the hell knows?
That's pretty easy: you cross the border, the border guard sees recent stamp showing you traveled to r---ia and you don't fit the profile of someone having a family there, which trip the alarm to look into you slightly more than usual (in normal days the only thing border guards do is check that passport is not forged).
Then the obvious question to "where you exactly you have been there and what for" will come out unexpectedly for you and you will start making up a bullshit story on the spot (after being on the road for the last whole day) and spit out enough signals to look into you even more.
At this point you are taken to a room for a talk and asked to show pictures in your phone, hotel confirmations, train tickets, etc, which obviously show you have been where you are not supposed to and you lying as well. If you panic and refuse to show the phone contents, you are likely just barred from entrance on the spot and put on a list. If you do show the phone contents and there isn't anything noteworthy, but your story is not coherent enough (or the opposite: coherent enough to the point you prepared for it), you get at date with a three letter agency who is supposed to not rely on feelings.
The "how do they know" is always a bad idea as long as people need to know and your actions leave extensive digital papertrail.
That's ridiculously stretched. Russia is huge. Plenty of choice of "where you've been to". And you absolutely don't have to answer anything about where you've been to in Russia, because it's actually not their fucking business. Worst they can do is not let you into Ukraine. Surely, theoretically they can illegally arrest you, and torture you, and whatever, but doing that to a USA citizen (or whoever you are) on a basis of slight suspicion you could've been in Crimea or some other territory they consider theirs inside of Russia, well, that's risking way more than you do.
Plenty of people have two passports for this reason.
Lots of things in immigration require you to honestly self-report and have no mechanism to validate at the point-of-entry. For instance, many countries without visa requirements will still ask about criminal records in your native country, but have no access to the data to verify if you tell them you've never even had a parking ticket.
But if you get caught later down the line, then they are going to use your lies to make life very hard for you.
Using two passports is not always the solution. As a dual citizen of two countries, I also have two passports. A couple of years ago I travelled to Malta with the passport from the country I currently live in. One week later I flew over to Israel, where I used the passport of my birth country. I was extensively interrogated at the border, because apparently the system flagged me to the border officer as there was no record of me entering Malta with the passport I attempted to leave the country with.
Interesting. But, TBH, it seems pretty obvious in the hindsight: it's a huge red flag that the travel history in the same passport is not self-consistent (as a result of using different passports for entering and leaving the same country).
I always wondered if this would get flagged somehow. I've never tried it myself, but I've thought about doing it.
What I was mentioning though is that countries will let you have a second passport from a single country if you have issues like this, to avoid showing the stamps in one passport.
Just to clarify, a prison sentence doesn't actually mean you'd be incarcerated, Russia for example routinely sentences foreign nationals to prison without the means to getting them arrested
