Majoru Oakheart
Adventurer
That seems like an exaggeration; 2 PL +0 encounters together > PL +4, which the DMG lists as the top end of 'hard but doable'. IME you have to get up around PL +6 before you're likely to actually defeat the PCs.
Not really. I've actually ran 2 encounters together. It resulted in a near TPK. It's possible, but it's certainly more difficult than a PL+4, which I've run before as well.
The problem is the number of actions they get. 5 enemies who are level 10 are a PL+4 encounter for 5 level 6 characters. That'll be very hard. If you have 2 PL+2 encounters for a total of 10 level 8 creatures....it's very likely that the PCs are all dead. It doubles their damage output each round. It double the hitpoints of the encounter. It's likely that the hitpoint total nor the damage total of the level 10 monsters is actually double that of the level 6 monsters.
The number of monsters is actually MORE important than the level of the monsters you use.