Your highest level of spellcasting proficiency from any source is your level of proficiency for all spellcasting. That is the case under standard rules. It is still the case if you use Dual-Class.
One minor note though, while they would use the highest spell casting proficiency they have access too, remember that their spell pools are still different. For example, a Wizard/Summoner would have both a set of prepared wizard spells and also their summoner spells, both different from each other. To my knowledge, these spells are not interchangeable either.