Yes, that is possible and what people did when S2 first came out. But to give more detail on what others already wrote:
Technically S1 contains half the cars & tracks, which does not sound too bad.
In reality S1/S2 cars or tracks get used together.
For example look at these 10 races here http://cityliga.de/index.php/en/season/schedule and compare which cars/tracks are S1 or S2.
As S1 user you could only join 3/10 races because of the way cars & tracks are mixed. That is just one random example but other leagues/public servers it is similiar.
This is a NZ opinion. I'd advise you wait till there is a good update that gives good Aussi/NZ racing. IMHO, our servers have moved on now ATM and there is bugger all good racing down here.
It's taken so long between updates that it will take a good update to get LFS things up and running again.
And servers like cargames boot us for lag cus distance, and ..........
I'd just wait and see what happens. And look at other games that are raced down here.
Sad to say this but there are no servers in this part of the world giving good LFS racing on a daily basis, and LFS works best online.
No one loves the AI, really, it's like loving dropbears......
It really hurt saying that but you deserve an honest answer!
And there are other games to play that are updated and have a following down here.
Sorry all, but that's an accurate review of the current Australasian situation.