You can't rank your proposals as a GSoC contributor. If you feel very strongly that you have a preference you can state that in that particular proposal. In the very rare chance
that both of your proposals are selected by orgs, the org that ranks your proposal the highest will be the one you would work with in the summer.
However, if that org happens to not be the one you wrote that is your # 1 choice then the org you did mark as your #q choice can mention your preference to the org that ranked you higher and the org can decide if they really want you for the project or if they had another good candidate and in that case they would let the other org have you.
This is not. something you should stress about. Very few people have proposals selected by multiple orgs (usually less than 12 a year).