I'm assuming this is a cash game, because that makes a difference. In a tournament, I will demand that Two Pair Guy table his hand to get the pot, 100% of the time.
If I'm playing cash at a casino or someone else's home game, I will not say anything during the hand. Yes, it's very possible that Two Pair Guy is angling, but making a comment when cards aren't tabled is an OPTAH violation. Once the hand is over, I would say something to the mucker.
In my home game, Two Pair Guy isn't getting shit until he tables his hand.
This is basically my answer too. Not my business to tell another player whether to table his hand, but that doesn't mean I can't be involved in another way afterward.
However, at a casino, I'm invoking IWTSTH ("I want to see that hand.") to force V1 to show his hand. What I really want is to ensure that V1 actually did have two pair, which I'm pretty sure is what
@Schmendr1ck is trying to do with his home-game rule. If he didn't have two pair, but his declaration caused V2 to muck, that entitles V2 to the pot, according to at least Robert's Rules ("The Showdown" section):
2. Cards speak (cards read for themselves). The dealer assists in reading hands, but players are responsible for holding onto their cards until the winner is declared. Although verbal declarations as to the contents of a hand are not binding, deliberately miscalling a hand with the intent of causing another player to discard a winning hand is unethical and may result in forfeiture of the pot. (For more information on miscalling a hand see “Section 11 - Lowball,” Rule 15 and Rule 16.)
This rule does specify "
deliberately miscalling," but I think that's too wishy-washy. It's an open door for dishonest players to legitimize their angle-shooting by simply lying again. IMO, if you miscall your hand and cause another player to muck, you forfeit the pot, period, with only extremely rare exceptions for the greenest of noobs (and TBH, not even sure about allowing this exception, especially in a public cardroom). This may occasionally cause someone to lose the pot through an innocent mistake, but that's the price you pay to secure your game against unethical behavior.
Aside from enforcing the rule, I want to know if the guy lied because it's going to have a big impact on how I treat him from then on. If he has the two pair, great, nothing to see here. If he doesn't, I'm going to tell every single person who sits down that the guy in seat 4 is an angle-shooting piece of shit. Ideally he'll just leave.
And if it's at a home game that I host, I'd immediately rack him up and ban him from future attendance.