Combo shells are a type of theory shell read against some combination of abusive arguments. That is, Argument might not be abusive read alone, and Argument might not be abusive read alone, but reading Arguments and together produces some abuse story.
For example, suppose that the negative says the affirmative should not get access to 1AR theory, and that the affirmative should also not get RVIs. The affirmative might read a combo shell on this, saying that these two arguments combined deny the affirmative access to offense on the theory layer since they cannot read their own shell and they also can't get offense without the RVI. Notice how the abuse is conjunctive – denying the affirmative access to 1AR theory might be fine on its own, or denying the affirmative access to the RVI might be fine on its own, but when combined, these two arguments become especially abusive.
Strategically Deploying Combo Shells
Combo shells can be strategic since they can be very difficult to respond to if there is a genuine abuse story. Your opponent, after all, would have to justify why their combination of arguments is somehow good under competing interpretations.
The main goal when reading a combo shell should be to generate a convincing abuse story. Whereas other theory shells might have many shorter standards, combo shells should generally have just one thorough standard which extensively explains how the combination of your opponent's arguments are abusive. Many combo shells will make some type of "infinite abuse" claim, which is to say that their opponent's practice makes it impossible for you to win the round.
Remember that your abuse story needs to be conjunctive; that is, it needs to rely on how your opponents arguments combined are abusive. One common mistake made when running combo shells is for debaters to justify why each practice your opponent is doing is individually abusive, but they fail to prove why the combination of arguments is abusive. If your combo shell is not about some combination of abuse, it would be easy for your opponent to respond to the individual abuse stories of your shell just like you read multiple theory shells, which defeats the strategic value of reading a combo shell.
When you are reading combo shells against some combination of theoretical arguments, (i.e. a meta-theory combo shell), you need to take extra consideration to make sure your shell is read against some combination of abuse. Suppose that the 1AC justifies that they get 1AR theory. The negative, in response, reads the shell "Interpretation: The affirmative must not justify that they get access to 1AR theory." The standards of this shell are all reasons to reject 1AR theory. Does something seem off about this shell? In reality, this is no different than simply reading reasons to reject 1AR theory, except in shell form. But when done this way, could the negative actually win the round for simply proving why the affirmative shouldn't get 1AR theory?
In response, the affirmative should claim that in order for the negative to win their shell, the negative needs to gain access to RVIs, since the negative's shell is functionally a counter-interpretation to the affirmative's implicit shell that they should get access to 1AR theory. This is an important, though technical point about theory debates. Theory interpretations that are directly answering some other theory argument on the flow technically need an RVI in order to gain offense.
In the context of combo shells, however, you can avoid this issue by reading a theory argument on some conjunction of theoretical abuse. Since your theory shell is no longer advocating for just the opposite of some theoretical argument but is now a meta-theory argument about the practice of reading some combination of arguments, the RVI would not be necessary to win the shell.
Responding to Combo Shells
Responding to combo shells through a counter-interpretation can be especially difficult if your opponent's abuse story is actually true, since you would need to come up with some proactive reason why reading your combination of arguments is a good norm for debate. With that in mind, you should take more of a deflationary approach when answering combo shells.
First, you could contest the paradigm issues. Winning drop the argument can be especially strategic since a combo shell will be read against some specific arguments in your case. Reasonability can be another good option if you can think of a convincing brightline with combo shells.
Second, you could argue that theory should be evaluated under an in-round abuse model instead of a norm-setting model. This is a more technical part of theory debates, but the norm-setting model of theory argues that the purpose of theory debates are to set good norms across all rounds, whereas the in-round abuse model of theory argues that the purpose of theory is to mitigate abuse in this round, specifically. If you win an in-round abuse model of theory, you can answer the shell by providing ways that your opponent could have answered your combination of arguments. This would, in effect, be putting terminal defense against the combo-shell.
Third, you can read generic arguments why combo shells should be rejected, like they fail to set norms in the debate space because they are contextual to the round, or that reading multiple shells solve.