With the new -2 to movement rule, what is the proper way to use Double time while carrying a character?

10 (Movement) -2 = 8 (Carry ability)
8x2(Double time) = 16 (Grand total of movement)

OR

10 Movement x 2 (Double time) = 20
20-2 (Carry Ability) = 18 (Grand total of movement)

Which is correct?

DT says "unmodified" speed, so you double that first then -2. So, a 10 will give you a 18 square swing.

Even without the feat saying double the unmodified value it would be the same. goes along with RS/Charge and carrying it would be half-2.

The ruling is that you double it and then reduce it.

This is something with which I seriously disagree. I don't think that the -2 should be applied at all.

You move it "up to double its unmodified speed value" without actually touching the speed value.

Your speed value stays the same. It isn't written the same way as Charge or RS which specify that you halve the speed value.

maybe they will errata it so that transporters dont get the -2 to movement.

That would not be an erratum. That would just change the rule.