i have build an universal app
i have assets that are for both but i have a couple assets ( embeded movie's ) that are only for iphone or only for ipad.
is there a possibility to make 2 builds 1 for iphone and 1 for ipad so that the iphone app doesn't grow to large?
No.
An universal app always contains resources for both platforms. You could of cause build an iPhone app and an iPad app from the same XCode project, but they would not be universal.
To do so, duplicate your existing universal target, open the build setting and set the targeted device family to iPhone, on your old target set the targeted device family to iPad, or create a second copy and set it's target family to iPad.
Open the target's Copy Bundle Resources build phase and delete everything, that's not needed for this targets platform.