Hello MickR33,
You can use two pumps if you want, but for me that's waste. Not only is it a waste of electricity running two pumps instead of one to do the same job, doubling the electrical cost. But it's also a waste of materials. Instead of having two pumps tied up in one system, two pumps could be running two different systems. If you designed it like in either option I gave you, there would be no need for a second
pump.
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Any idea how fast i want the pump to be pumping into the tubs to circulate the nutrient solution? the tubs are 45ltr ones i think and the pump i have at the moment i think is just a 350ltr an hour pump and i have it turned to maximum.
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There isn't any real defined standard for circulating water or air. Circulating more or less just means not remaining still/in one place. But I typically use a standard rule of thumb "1 hour" for calculating circulation. Meaning to circulate the entire volume in 1 hour. For water once an hour, for air 5 times per hour. Here your circulating water, so I wont go into calculating for air. You say your tub's are 45 liters (about 12 gallons) each. So I would want circulate the entire 12 gallons through the system every hour. You have 2 tubs for a total of 24 gallons to circulate each hour.
You said your pump is rated for 350 ltr (about 92.5 gallons) per hour. Normally you would need to take into consideration Head Height (the back pressure/weight from the water when pumping it above the water line). But because of how you have the system set up, your not needing to pump it more than one foot above the water line. So the head height doesn't really come into play here. So assuming your using the maximum size tubing the pump is designed for and not otherwise restricting water flow from the pump, your pump should be able to put out around 92 gallons per hour. 46 gallons (173.8 ltrs) per hour to each tub.
In other words, the way you have the system set up, and with the pump you have. That pump is capable of circulating the entire water volume of both tubs more than 3 times per hour (3.83 actually). There will be some natural points of restriction of water flow from the "T" and elbow connector's etc.. But their minimal and you don't have many. So even rounding down from 3.8 times per hour to 3 times per hour to compensate for the "T" and elbow connector's. That pump is more than capable of providing enough water circulation if set up right.
P.S.
Ask anything you have questions about, that's what the forum is for.