I have a layer that is scaled at 50% and one that is scaled at 30%. I have positioned them to look sweet together and now I want to make that thing just a little bigger. Here's my problem: Because they have different scales, selecting them both and pulling on a corner makes them grow at a different rate. If I make them into a pre-comp, then rescaling up will make me lose resolution.
Any way I can select 2 things and modify then at one object for the time they are selected together?
That's what Null layers are for. Add a null layer, parent both your layers to it and then resize the null without loosing proportion between your layers.
You will not loose resolution in precomps if you check the "Preserve resolution" option in the Advanced Composition Settings.