I think it's in that huge thread of custserv responses, but the powers gained from paragon multiclassing count as powers of your second class.
'Your second class' is apparently interchangeable with 'your class' when replacing lower level encounter & daily powers with higher level ones (upon leveling up).
Although I've largely given up keeping track of custserv responses, since they fail at consistency.
This is correct. I'm the one who got that customer service response.
So one advantage of paragon mutliclassing is that you can 'trade up' what would have been your paragon powers for higher level powers in your second class. You can't do that with paragon path powers.
That said, it's really only an increase in flexibility. In order to trade up powers from your second class, you have to forgo trading up powers in your primary class.
At any rate, that does get you a lot closer to a 50/50 split by the time you hit level 20. Minus class features of course.
I really think the bang for your buck in multiclassing is being able to combine powers from very different classes. I think it will result in some pretty cool synergies that aren't available otherwise.
But, as always, your mileage may vary.
AD