A new mechanic in TTT2 is that it's possible to keep the opponent's character in while they try to tag out. As such it's possible to hit both characters simultaneously, aka "happy birthday" or "BOGO", as the tagged out character tries to escape. While a character is trying to escape, they take greatly reduced damage preventing MvC style kills (where assists take 150% damage) as in TTT2 any KO results in the end of the round. This shouldn't discourage you from going for kills as a solid lockdown string can quickly end the round.
Before I get into how to lockdown your opponent, I should first explain how tagging works. When the opponent is standing, their character will run out and their secondary character will run undefended. Since you can not block while coming in, you can choose to come in attacking with your standard options or come in with a running attack (i.e. f,f,f+4 for generic slide, f,f,f+1+2 for acrobat roll). When the opponent is grounded, the tagging out becomes much more risky as the tagging character has to crawl away making the oki similar to what we're already used to in T6, namely FDFT type roll away.
When the opponent is at the wall, the options for tagging out become much riskier. If the opponent is caught while tagging out, you can W! the changing character and keep the character in by continually hitting them. If the character is also KND while trying to tag out, they can be stuck in a perpetual crawl to the wall where they can then be re-W! and KND. This obviously is the state you want your opponent to be in and you can abuse this weakness by using strings of low-ish hitting attacks that W!/KND.
Unlike MvC 3 where you can use advancing guard to push the opponent away or supers to cover a tag, once their background character is in a combo state, the only way to stop you is to block the options and punish accordingly. It sounds easy, but is incredibly hard to do. Now there is the "Tag Crash" system, performed by inputting 2+5 while in a KND state, but even that cannot be performed when being locked down during a raw tag. As the game matures and players become more accustomed to this mechanic, their defense may improve but I'd argue that we're going to see tag out rushdown kills for a long time.
I make this assertion based on two main things:
1) When your life is dwindling down during a round, you're more inclined to panic and make simple mistakes
2) A ton of characters look very similar and due to netsu rules, people will be choosing characters with some relation, naturally also look similar (i.e. Asuka + Jun, Mishima teams, etc.)
A few other more minor reasons might be:
1) People prefer certain color palettes and most likely will be coloring both of their characters, subconsciously, very similarly
2) Apart from character appearance, many characters share a common height, possibly causing confusion when both characters are standing
Any type of attack that leaves you with a WS+4 option is your best bet (e.g. b+2, f,f+4, d/b+3) for actually comboing the opponent. A simply BnB like f,f+4, WS+4, d+4,1,3 is actually very strong, not only does it re-W! the tagging character, but CH d+4,1,3 is NCc and will W! the point character as well. Once both characters are W! and B!, the match is essentially over as one 50/50 will most likely KO the tagging character.
For advancing attacks, workhorse moves get the job done such as 3,1,2, b+4,3, d/b+2,1,2 and since all of them W!/KND, the pain train keeps rolling on your opponent. If you watched my TTT2 matches, you may have noticed me using d+3,4 a ton, I was using it mainly on anticipation of a raw tag. The d+3 would catch them BT thereby guaranteeing the final 4, also at the wall it would cause a wall bounce property keeping the tagging character in even longer. This would set up an advancing mid KND/W! to go for the kill.
If you anticipate not being able to catch the opponent, but will hit the incoming character, qcf+3 is good for eating tag jabs and jumping attacks, iWR+2 for slower attacks (which can also catch the tagging character for a FDFT KND and 50/50 follow up), b+4,2,1 isn't good for catching the tagging character but is great for interrupting the opponent's attacks. Finally d/f+2 or qcf+2 if you think the opponent is going to do nothing, but will probably be rare in the future.
The only example I can find, of myself, performing a tag lockdown is here. Since Dragunov was so low on health, a raw tag was obviously coming up. I use d+3,4 to apply pressure to Dragunov while also using it as an option select to eat any incoming attacks, since it's NCc. Since Dragunov is now spiked by the 4, he must once again roll away FDFT which I use d/b+3 to once again catch him. This would normally guarantee a WS+4, d+4,1,3 follow up but Dragunov luckily rolls to the opposite side of me ending the string. I still finish up with d+4,1 as a CH confirm just in case AK attacks with a wakeup 3/4 or as a possible roll catch.
Why I think d+4,1 was nerfed so fast:
In my opinion, the NC d+4,1 nerf was because of Dragunov's ability to keep the tagging opponent in so easily. I'm not saying that other characters won't have as easy a time, but I am saying that d+4,1 was forcing the opponent to block the final attack. That sounds a little goofy and granted they could probably still duck the final hit and launch punish, but using the reasons I mentioned earlier, d+4,1,3 is 100% guaranteed on an outbound character and the point character cannot stop it once it's begun.
If the opponent panics and tries to defend his tagging character by attacking, both of his characters are getting KND/W!. If the opponent panics and doesn't do anything, his tagging character is getting KND/W!. If the opponent stays cool and ducks the last hit, his tagging character is still getting KND/W!. It was a nice little lockdown while it lasted.