Only been playing with the core set for a little over a week, but have the rebel aces and imperial aces expansions on the way... I have been playing trying to keep my Tie fighters in close formation but wonder if it might be more advantageous to split up and make the xwing pick a target to follow... advice? What are the pros and cons to formation flying and dividing your forces?

You want shots while preventing him from getting shots. Consider this, if you fly them as a pair you get more firepower but then the X-Wing only has to worry about one target. If you break them up you get less firepower but if the X-Wing goes after one then the other should be able to after the X-Wing unhindered.

Any way you fly it the X-Wing gets to pick one TIE Fighter to focus on although you may not know which right away.

If you fly as a pair you're likely to get attacks, or not as a pair. If you "win" you are more likely to keep both ships when you lose one the other is likely to be in a similar position as the first.

If you divide your forces you'll know who the X-Wing is going to attack which lets the other concentrate more on making sure the X-Wing is getting shot at. Your odds of losing one go up but even when lost the other should have an advantage over the X-Wing as it now needs to go after a target that is coming at it from a different direction.

Welcome to the game! My advice? Go play for 2 months to a year without reading any other threads on this forum. Too many bitter old souls who will tell you which ships are terrible (nearly all of them) and which tactics are unsportsmanlike (anything they don't like).

To your question: Flying in formation is a guaranteed way to get lots of shots on a target... or none. Your goal (as StevenO said) is to ensure you get shots while denying shots to the enemy. If you can't deny shots, at least force your opponent to divide up damage by attacking different targets each turn.

I would say flying in a loose formation is good to start with. Toss in Howlrunner and you want a tighter formation. Otherwise you will find yourself wanting to spread out a bit more as you get comfortable with controlling multiple ships and still getting guns on target.

Flying with just the TFA Core I've found that the TIEs work better when they split up. Invariably, if/when the X-wing turns, the non-pursued TIE gets the shot(s). Meanwhile the pursued TIE will eventually win the turning game and get out of arc on the pursuing X-wing. Exact copy of actual dogfighting tactics when flying with a wingman. Usually this means the Reb player loses.

P.S. I suspect that as I add more ships to the fight, this tactic will no longer be as effective.

Epsilon Leader is a good reason to fly in formation as well. And epsilon leader comes with the core set.