I’ve instituted the following simple house rule into all of my games to solve tricky situations wherein players attempt to place a Bag of Holding inside a Portable Hole another Bag of Holding.
If the player attempts to place a Bag of Holding into another Bag of Holding, the two simply repel each other like magnets, making it impossible to do so – or at least exceedingly difficult (DC 30 Strength check). The same goes for placing one Portable Hole inside another, or for any combination of Bags of Holding, Portable Holes or any similar items.
I typically eliminate the Portable Hole from my games entirely, since it is a silly item. I also allow Bags of Holding to be carried into the extradimensional space created by a rope trick spell, and rule that when a Bag of Holding is split open its entire contents spill out as opposed to being lost. This actually saved a character’s life who managed to keep a wagon in his bag, which he hid behind to prevent a drow lich from having line-of-sight to cast disintegrate on him.