Saying they are both "in hell" is an over-generalization. That's like saying a person sitting on the beach in sunny Florida and a person freezing to death in a Siberian prison are both in similar situations "on earth". We know that Heaven is a vast, vast place with different levels and glories, as each level gets closer to God. Hell is probably similar.
Yeah, this part just doesn't seem to sink in here. "Hell" can refer to hell of the damned or else to Limbo (as it does in the Apostles Creed). Church has not technically defined
Limbus Infantium, just rejected the Jansenist assertion that it's Pelagian. So one can claim that infants burn in Hell if they like, but St. Thomas had it right. As for
Limbus Patrum being a difference "place" from
Limbus Infantium, there's absolutely no Church teaching on the matter. Could very well be the same place, just with different occupants at different times. In fact, there are a minority of theologians (who have not been condemned) who claim that Hell isn't a specific place but can be anywhere and everywhere. There's very little the Church has defined on the matter.
We know there's an eternal Heaven.
We know there's a Purgatory that will go away at the end of the world.
We know there's an eternal Hell of torments for the reprobate, where each individual suffers in proportion to his sins (that's part of one dogmatic EENS definition).
We know there was a
Limbus Patrum in which the OT just resided until they were liberated into Heaven by Our Lord's descent there.
Beyond this, there are a variety of opinions. St. Augustine and a few modern holdouts believed that infants who died without Baptism went to the Hell of suffering. St. Thomas thought that there is a
Limbus Infantium where there's perfect natural happiness (I agree with him). Then there's speculation about a lot of other matters. We saw Dante putting "noble pagans" in Limbo (I agree with him on that also). St. Ambrose wrote of martyrs who died without Baptism as "washed but not crowned", where they could not enter the Kingdom but were somehow washed. Do these go to Hell proper or do they go to Limbo? Or are they actually baptized invisibly by angels (per St. Cyprian for instance). If, as per St. Thomas, God would send an angel to enlighten infidels, can't this same angel also baptize them? Theologians after Trent (like Cano) wrote of infidels who could be justified, but not saved. Where do they go when they die? There are so many open questions beyond those few points above that are simply not defined. And BoD is actually in this same category of speculations. There's no evidence it was revealed. For 700+ years, with the exception of some Jansenists, nearly every theologian believed in the
Limbus Infantium described by St. Thomas. And yet the Church has not stepped in to positively defined
Limbus Infantium as Catholic doctrine, and it's not considered error to cling to St. Augustine's opinion ... unless you also hold that Limbo is Pelagian (which was condemned by the Church).