It is possible, but requires a bit of manual configuration right now:
You can use the X2 chain (E5-by-X2) and then manually include the cross-signed X2-by-x1.
We aren’t currently offering this chain in our API because we have some concerns about compatibility with existing ACME clients. We are evaluating how we can offer it in the future.
Can you tell us more about your use-case? We are interested in hearing about why people want this configuration