How to always keep steam turbines running at maximum performance using nuclear power?
It's hard to say where the bottleneck is because you haven't shown any of the mouseover info displays for the turbines, exchangers, and pipes, but here's some general advice:
First of all, your setup is missing the bonus for multiple nuclear reactors. You must place them exactly adjacent to each other, with no intervening heat pipes. Once you do so, you will have exactly twice as much heat produced.
Try to avoid cross-connecting heat pipes — each one should be a direct line from reactor to heat exchanger, and the only time three or more pipes should meet is if you're branching to feed more than one heat exchanger. (I'm not sure how much this will help, and it might change in the future, but my understanding is that in current versions heat pipes work pretty much like fluid pipes.)
Simularly, make sure that the water pipelines are not cross-connected — for each offshore pump there should be a group of exchangers using its water with no overlap with other pumps. If there are cross-connections then water may flow in useless directions, reducing flow in the useful direction.
In general, not enough offshore pumps can mean not enough water, but you don't have enough heat exchangers to need more than one, here. But you didn't show how many you have, so maybe adding a second one if you have only one might help if your water pipeline is not too efficient.
steam turbines can be linked together like steam engines, but doing this almost always ruins the performance of the turbines.
This just means that you don't have enough steam in that pipe to feed that many turbines. The total output will always be the same or better — there are no losses in a heat/steam system, only flow bottlenecks.
This is an older question, but your 2nd steam turbines are always going to be limited because the heat exchanger only can create 10MW worth of steam - but each steam turbine is 5.8MW capacity. So the second turbine will always be limited to only 4.2MW production.
You can get around that by having more heat exchangers and a "shared" steam process between them.