Managing 2 or more kids at the pool by yourself?
Personally, I would enroll them both in swim lessons. I'd choose lessons that teach them how to get to the edge and climb out, not lessons about perfect strokes.
If I chose to swim with them both at the same time without another adult (which I'm not entirely sure that I would do - I'd probably only use pools with lifeguards and other adults nearby) I'd put the younger one in a life jacket/vest. If the older one wasn't a good swimmer (not sure what you mean by beginner swimmer since I've seen parents use that term for a wide variety of skill levels) then I'd put a life jacket on the older one, too. I'd tell the older one that we have to stay close to each other (arm's length) or else we have to leave (and then follow through if the older one swims too far).
Here is a guide to life jackets from the Coast Guard: http://www.safekids.org/sites/default/files/documents/how-to-choose-the-right-life_0.pdf
I didn't feel comfortable taking mine alone at that age, in fact this summer is the first time I have taken them both and they are 6 & 8. Part of that though is that till now the 6 year old has wanted to be in the baby pool while the 8 year old is quite a good swimmer and wanted to be in the big pool and due to the way our pool is set up that meant I couldn't see her. Now they are both OK in the big pool so DD can do lengths and check in with me at the shallow end each time.
When they were smaller I found it much easier to go with a friend, even though this meant more kids it made it easier to supervise them. We both had similar aged children so one of us would be able to play in the shallows with the younger ones while the other went to swim with the older ones. It also meant there was another adult around for those moments when you need to take one off to the toilets.