Ok, now I think I get it.
I'm assuming that your 20A outlets are 120V. As Wberry said, 2x1500W elements will draw 25A, so your outlets are not enough. You'd need 2x40A outlets (or 4x20A), to be able to run that load with an adequate safety margin.
Now, to your original questions:
Theoretically, you can do it either way. Generally speaking, all measuring and control devices are designed to be able to drive 5 standard loads from any output, which means you shouldn't have any problems running 2 SSR's off a PID. That said, if you choose to go this route, MAKE SURE FIRST THAT'S THE CASE with your PID and SSR's, so you don't have any nasty surprises later.
Personally, I would use 1 SSR for every 2 elements. Running 1 SSR per element has no real advantages, other than allowing you to use cheaper, 20A SSR's (instead of 1, 40A unit), and eventually allowing you to use independent power circuits for each element (notice that I said circuits
, not outlets. I wouldn't run the 4 elements off of 4 20A outlets if they shared the same circuit). If you don't plan on doing that, you're just adding potential failure points to your setup.
Finally, I agree with Milldoggy in that 240V would be a much better option for you. I would go for a 40A circuit though, as 30A is a little too tight for comfort. That way you have the option of running the elements in series, running each pair off of a 120V branch, or running each pair off of the 240V, but using the neutral as a separator (a little more expensive, as you will need twice the breakers/fuses, but, at least IMO, the best option, as it keeps the loads balanced among the 2 phases).
Hope it helps...