Could someone clear up a question about summoning for me?

[Kristev]

Hello. I don't know what the rules are because it never came up before, but can I use Summon Nature's Ally 2, and on my next turn, Summon Swarm, or will the swarm spell end the ally spell?

[Melkiador]

The spells can coexist.

I should note that the timing can feel a little weird though. Those spells have a round casting time. This means you cast it on the first round and it comes into effect at the beginning of the next round. In that next round, you then still have all of your regular actions and could cast another round spell, which itself wouldn't come into effect until the following round.

What you might be thinking of are some spell like abilities that limit you to one summoning at a time from that ability. But you usually still have access to additional simultaneous summons from spells that are outside of that spell like ability.