Even though more is better, 10 ga is overkill for just 2 lights.
Now with that being said, if you have one of those soldering guns that are about 100 to 140 watt, you should be fine. I typically strip back an inch on either side, slide my heat shrink on, then push the wires together end to end then heat and solder. Makes a good connection and doesn't get much thicker than the original strand of wire.
'88 YJ Laredo, Hardtop
4.2L Auto 190k miles
3" Body Lift, Rancho Shocks, 31" Wranglers