It's in the multiclass spellcasting section of the last playtest packet under Spells Known and Prepared. It even gives an example of a Bard 4 / Mage 3 multiclass where the bard only knows two cantrips and three 1st level spells and says the mage can have 2nd level spells.
Ah the example in the leaked Alpha?
Yes, reading that example would seem to back up your interpretation. Although its still not 100% clear cut. If it is the intention that you are capped with low level spells, caster multi-classing is effectively going to be dead in water as an option again.