The most trouble I've had is usually with white being smaller (in all materials). You can drop a strand with the larger, or add a strand with the smaller, but it may not fix the problem completely (may add/remove too much).
Right after you start your second loop, constantly check to be sure you have even tension on both bundles. If you see one bundle getting some slack in it, cheat it out by adding a little extra twist to that bundle as you go (don't try to get it all out at once).
Chad