(Please ignore the esoteric paraphernalia in the background. Necromantic rituals tend to come with certain requirements.)
An alternate way to begin to make it into a story: for whatever reason (perhaps because they're too tightly bound together by this point to avoid it, perhaps just from sheer bad luck), the trip back deposits them both in the Kyuubi's body - neither one under seal, and presumably neither one willing to just sit back and let the other have free rein... and a fight for dominance quickly ensues. (Which, just as something to consider, might look rather odd from the outside.)
Naruto may not have ferocity to equal the Kyuubi, and he certainly doesn't have comparable levels of chakra - but you'd have a hard time arguing that he doesn't have the strength of will to hold his own in a battle like that.
So, when it's all settled out, you have the two - presumably rather uncooperative - minds in the body of the demon, neither one able to definitively wrest control away from the other for any long period... and then you begin to deal with the consequences. Which could be quite varied, depending on exactly how far into the battle things had been when the minds from the future arrived - or even whether the attack had begun at all.
Possibilities?
An alternate way to begin to make it into a story: for whatever reason (perhaps because they're too tightly bound together by this point to avoid it, perhaps just from sheer bad luck), the trip back deposits them both in the Kyuubi's body - neither one under seal, and presumably neither one willing to just sit back and let the other have free rein... and a fight for dominance quickly ensues. (Which, just as something to consider, might look rather odd from the outside.)
Naruto may not have ferocity to equal the Kyuubi, and he certainly doesn't have comparable levels of chakra - but you'd have a hard time arguing that he doesn't have the strength of will to hold his own in a battle like that.
So, when it's all settled out, you have the two - presumably rather uncooperative - minds in the body of the demon, neither one able to definitively wrest control away from the other for any long period... and then you begin to deal with the consequences. Which could be quite varied, depending on exactly how far into the battle things had been when the minds from the future arrived - or even whether the attack had begun at all.
Possibilities?