Moving two elements from series to parallel
You can't just connect them in parallel.

If you're fluent with this stuff, you can drive each one directly from 230 V, but with PWM so that the average power doesn't exceed 600 W.  There are various ways to do this, including triacs, but there are issues beyond what we can reasonably get into here.  You have to consider what happens when the system screws up and applies the full 230 V.  In that case the power dissipation will be 4 times intended, or 2.4 kW, for a short while.  That will damage the element quickly, and possibly other things around it.

Another option is to use a transformer to make 115 V, then drive each element independently from that.  That will work fine with less chance of catastrophic failure, but will require a big, klunky, and expensive 50 Hz power transformer.