Aye its going to be erm fun.
I have 2 strike teams ready, its around the middle of March and I have some experienced members in both teams. Gonna hit it with the first team, and if not successful the second team is going in. Also i'm not hitting the one near my main base, keeping that one around for milking the supply ships. The only issue is I'm not sure the race of the other base, if its muton or something hard then I may change my mind haha. If its a squid wipe or two, so be it, either way its going to be fun watching!
If you find and locate the second base, if there is one, i would suggest to not storm it, but go for the floater base nearby your starting area.
You know the race, you know which terrorunits will occur. The second base already could be a Sectoid (PSI) one or a Floater/Sectoid Mix (No PSI) in both cases you would also face Cyberdiscs in close quarter combat, which can also by killed via Shotguns (as suicide solution), but the best way to kill those is 2 incendiary grenades. They vulnerable to fire.
The longer you wait with storming the alien base the better the alien weaponry will be, meaning the aliens in January will mostly have Plasma Pistols and Rifles, later on they will get loads of Plasma Shotguns which makes fighting them seriously unpleasant, since alien bases are a crowded space and tightly packed.
Also the aliens are earlier aware of your troops locations, the 20 turn rule only applies towards Very Small UFO's.
Small and Medium take 14 turns instead of 20. Large and Very Large only 10. And in Alien bases its only 7 turns.
I am just mentioning this because the aliens totally act different once they know your location.
Alien Bases do not only generate Supply Missions, there is also a chance for them to generate, Alien Research, Alien Abduction or Alien Harvest Mission for the region they are in. Those mission will also be from a different race.
So a Floater Base, can generate a Muton Harvest Mission in June or so, otherwise it will just pick one of the currently active races, which so far have shown up.
I seriously advise to research Laser Rifles ASAP, latest before June, because you will have Mutons in June, loads of Mutons!! July is Muton only!
And you needs Laser Weapons for Mutons, the new Terror Units they bring with them are hard to kill without them.
Muton Berserkers and Armored Cyberdiscs are no jokes.
Sorry for the spoilers :-/