Just flip the phase on one of them. This is no different than if you mount 1 sub inverted.
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This seems very different to that scenario.
In fact, if they are sharing the same enclosure, flipping the phase will mean one is pushing when the other is pulling, negating the effect of having an enclosure, easily going into over-excursion. With the subs out of phase, you'll also get very little bass, because what one plays will be cancelled by what the other plays. (except at certain frequencies where they might be in phase due to path length differences)
It might work OK with them both in phase, but you will encounter phase problems if the path length to your ears is different for each sub. You'd need to have different TA on each sub to correct this.