Gel electrophoresis ?

Theoretically, you can.

I assume you have some background knowledge on 2D-gel electrophoresis so I won't go into the theory behind it.

When you sort it by charge first and then size (the conventional method), you would notice the proteins are all laid on a gel strip, where they will migrate to their iso-electric points. Now, when you laid the protein mixture on, it is all on the SAME surface, yes?

Now if you turn it around and do it by size first and THEN charge, what happens to the proteins? The proteins are then on different surfaces (some would've migrated lower, some higher, etc etc), so how do you have a consistent way of moving them according to charge?

Bear in mind that the gel strip used for charge separation and the gel used for size separation are two different gels, so you cannot do size on one gel and then try to do charge on the same one, you'd have to move the proteins from the gel to an isoelectric gel strip, which won't be large enough to cover your entire PAGE gel.