The double battles are only part of the reason why the games is more difficult. There are also many other reasons, including:
1). It is way harder to OHKO opposing tems even with super effective moves, so you are bound to tank at least a few hits.
2). There's a billion tamer battles in a row, so in order to make it through you either have to backtrack a lot, or bring a lot of restorative items which REALLY sucks. The temessence vial doesn't help much, because you need to heal after almost every battle.
3). There's no EXP share (except Coward's Cloak, which you can only equip to 1 tem),