I use 2 remote on/offs connected by a very short piece of stainless braided(or a dual asa gun works to, in a pinch
). Chill your 3.5's before hand and keep the 20oz (or 24 oz if you can pick one up) warm. fill with the 20 oz upside down so you get liquid, or anternatively install a siphon. Always weigh the 3.5 oz before and after, because you will usually overfill and you'll probably want to let a bit out.
Edit: The only problem I have with my setup is I blow a lot of oring, because there's no way to vent the lines before I unscrew a tank. I'd like to add an on/off vent,but haven't gotten the parts yet.