For 2x 4500w elements running at the same time wouldn't you need to go with 50A instead of 30A?
I also think the common variants of 2x 4500w I see are:
1 - 2x independently fired elements for back-to-back batches.
2 - 2x fired-together elements for larger batches / quicker ramps.
I'm not sure which setup their 50A DIY kit schematic is setup for or which setup you're looking to do.
I ended up going with a bunch of parts from them for my 50A panel as well as some parts from other suppliers. I'm setting mine up for larger batches instead of back-to-back batches.
EDIT: in case you didn't see it here's their 50A DIY kit