Why do I have a separate limit for Credit Builder and Monese Boost?

This is because they’re slightly different products. Credit Builder is a term loan (0% interest loan) that will finish after 12 months, and

Monese Boost is a revolving credit facility, that you can use to boost your budget, and which will continue after your Credit Builder is closed.
Monese Boost can only be used when bundled with Credit Builder.

You can read more here