There isn't just one right answer.
It all goes back to how well you know your current decompression status.
The rule of staying out of the water for 24 hours if your computer fails is a nice, simple, conservative rule that doesn't require any calculation or thought or knowledge of your dive profile. If someone very literally followed that rule, they would stay out of the water for 24 hours if one of two computers they were wearing failed during a dive. Clearly that is nonsense, because in that case you have just as good information on your dive profile as you did before your primary computer failed.
The situation you described in your post is in between. You don't have perfect knowledge of your profile, but you do have enough to make a rational guess. And you are always free to pad your guess with as much additional safety factor as desired to account for the unknowns of your profile. Maybe you'll choose to stay out of the water for 24 hours. Maybe you'll add a couple more pressure groups to your assumed surfacing PG and extend your SI a bit more. Maybe you'll just assume you came up in the max pressure group Z. (Check it out ..... it isn't that big of penalty to make that assumption. Assuming Z rather than Q just extends your SI by 1/2 hour).
Like many things in life, it all comes back to using all of the available information. Your buddy's computer and your buddy's dive watch are bits of information that help you make a good decision. You shouldn't go diving in buddy pairs sharing one computer, but if your computer has failed, the info on your buddy's computer is one more tidbit of information that you can use to make a rational judgement.