Scenario 1, a bag of holding into a portable hole: No.
When the bag of holding is placed into the portable hole, a gate is opened and the two items are sucked into it and forever lost. Essentially, they consume each other. There is no mention of anything else getting sucked into the gate or if the gate allows other creatures and objects travel to the astral plane.
Scenario 2, a portable hole into a bag of holding: No.
When the portable hole is placed into the bag of holding, the two items and any creature within 10' are transported to the astral plane, and the two items are destroyed. So if you are in the belly of a beast while doing that, you, the beast, and the two items (which get destroyed) are transported to the astral plane. One could say you are still in the belly of the beast.
There is no explicit mention that all living and nonliving matter within 10', no matter how whole or how partial, is transported, which would be necessary for the "sucked through inside-out" thing you're describing. It only mentions the two magic items, and any creature within 10' of them at the moment of rifting, so you and the Tarrasque probably just appear on the other side intact.
From the SRD
article on the bag of holding
If a bag of holding is placed within a portable hole a rift to the Astral Plane is torn in the space: Bag and hole alike are sucked into the void and forever lost. If a portable hole is placed within a bag of holding, it opens a gate to the Astral Plane: The hole, the bag, and any creatures within a 10-foot radius are drawn there, destroying the portable hole and bag of holding in the process.
If Unsatisfied... Make a DM Ruling
Page 6 of the Dungeon Master's Guide:
Often a situation will arise that isn't explicitly covered by the rules. In such a situation, you need to provide guidance as to how it should be resolved. When you come upon a situation that the rules doesn't seem to cover, consider the following courses of action.
- If you have to make something up, stick with it for the rest of the
campaign. (This is called a house rule.) Consistency keeps players
satisfied and gives them the feeling that they adventure in a stable,
predictable universe and not in some random, nonsensical place
subject only to the DM’s whims.