Twin Alternater or big single Alternater?

I'm running twin Optima batteries with a split charge system due to my winch, fans, lights etc. It all works fine but nows the time to upgrade the Alternater. I have a fairly new one on there at the moment, just a standard jobby. The engine is a 4l v8 from a range rover in a defender 90.

Is it better to just buy another standard Alt and have a twin set up. Or to go and buy a bigger one and have a single Alt.

Any suggestions or advice would be brill, oh and happy new year!

Why do you think you need a bigger alternator? If you only have two batteries you have say 230 amp hours of capacity combined. It would be a waste of time putting another alternator on or even an oversized alernator. Without the battery capacity you probably are not even using the full output capacity of the existing alternator before the alternator sees the 14.5v limit and therefore no more amps. You probably have a 100amp alternator. This is more than adequate even with winch, lights, etc. If you want to improve you system add more battery power. Your alternator will then have some work to do.
Also, Optima is a waste of money. They cost a load of dosh and give no advantage

The last Alt burnt out around a year ago, I was running twin kenlow fans which didn't help too much. When winching at night, with 2x fans running and all the lights on it is alot of strain on the alt and from the display that I have in the cab it suggests that over time the batteries are draining so I really do need to do something with it. I do have a 100amp on there at the moment.

With regards to the batteries they are expensive but I had someone supply them to be very cheaply, from what I remember they were only around £90 each brand new. Brilliant batteries and are great for cold cranking. I even flooded my defender and used the cranking to get me out of the river, any normal battery would have died. No trees around to winch off. I would recommend them to anyone.

Well to me one big alternator would be the easiest option. There's slightly less resistance when its not under load than having 2, there's no need for a longer belt or more wiring and its one less thing to break.

If you go above 100amps you are likely to increase loads dramatically on your pulleys. 100amps is approximately 4 hp load. If you doubled this to a 200amp alternator it would put 8hp load through pulleys. Belt slip is highly likely to say the least.I would still suggest doubling the battery capacity as the best solution. You can then winch and run all your fans for twice as long.
Most batteries dont like to dicharge below 50%. so if you have 200 amp hours in your battery bank you have a useful 100amps of safe discharge. Double the bank to 400 amps and the same 100 amp discharge only drops your bank to 75% dicharged. A lot better for your batteries.
If I was you, I would consider a battery bank increase if you have the space and consider a smart charger that will maximise the output of your existing alternator.
Sterling smart charger regulators are very good

I use a numax 1000 amp bat connected to a 100 amp altinator and so far I have had
No probs running a winch 9.5 ti and 2 lightforce 170 spots (100 watt each )
So under max chat I am drawing 625 amps but this is a very rear thing for me
To do