I've only looked at doing this with an MK. The main problem was that when you cut the neutral bar in two, you end up being short of 2 plastic pieces to hold the 2 new ends of the neutral bars in position.
What stops you supplying the two N supply sides of the two RCD straight from the protected side of the isolator? Then use the two existing neutral bars as one for each RCD.
You only have a problem if you need two RCDs + some unprotected circuits - ie a 3 way split box.
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