If I was to run 2 Tempest Filters split evenly off one pump feed at 10000ltr/h would this then equal the optimum flow rate of 5000ltr/h through both Tempests?

Yes, splitting the flow rate from a 10,000 varipump through two Tempest filters is fine, as in theory it will provide a flow rate of 5,000 lph through both filters.

However, we recommend installing a ball valve before each filter so you can control the flow rate specifically to maintain 5,000lph through both filters. You can also control the flow via the Varipump controller if you need to turn the whole system down a bit. 

During cleaning please ensure you clean each filter independently and turn off the pump before closing the slide valves, to avoid any chance of running 10,000lph through one filter.

See page 14 of the instruction manual for more details on setting up multiple Tempest filters: https://evolutionaqua.com/pub/media/bss/productattachment/MasterTempestManual.pdf