On a Sunsynk 5.5kW mppt I've got 5x Canadian Solar 395W. I'd like to use the 2nd mppt to fit 5 panels on another part of the roof but I'm a little confused about balancing. Is the balancing on 1 mppt or across both?

So do I have to get another 5x 395W panels, or can I get 5x 450W JA Solar panels?

(The part of the roof with the new panels can only fit 5; I could possibly add another panel to the setup on the 1st mppt)

as long as they are on separate mppt's, it does not matter, I have nine 355 watt panels on one mppt and six 540 watt panel on the second mppt

Thanks!

That's the advantage of having two separate MPPTs. Each can manage a different type of panels and different orientation to the sun from the other.