Ah! Two things I considered as I got ready for work:
1). No need, thus far that I can think of, for an additional box of Descent 2e. Just when you insert an additional two to four heroes (not just an extra one, explained below), create an additional open group of monsters on the map to accommodate the additional heroes using the backs of the new monster cards. If reinforcements are allowed, you receive additional reinforcements. For example, if you are playing Encounter 1 of A Fat Goblin, reinforcements are a Goblin Archer from exit, and 1 from open group at the entrance. Additionally, the Overlord spawns 1 more Goblin Archer from exit, if able, and 1 additional figurine from the other open group at entrance. The overlord also draws additional cards at beginning for each hero as usual. Also, the overlord must place an additional treasure for every two new heroes that start on the map on squares of his choosing, so long as each square belongs to a different tile and is not exit nor entrance tiles. I'm sure someone will come up with a better suggestion, but for now this is the one that I offer as well the good note that you will not likely need to buy Descent 2e twice if you can get past being short defeat icons to place on the board, having not enough condition cards, and readily able to tally with pen and paper or separate game components beyond Descent 2e for health and fatigue.
2) If you very well must buy an additional copy, then you can have a party of five players. Substitute a white figurine for a new red one from the new box of Descent 2e when playing with a fifth party member. Also, open groups inserted as said above may instead be new figures from second box that are monsters from the current quest. Thusly, with a party of eight in First Blood, place 4 Ettins and 10 Goblin Archers on board.