Hi,
Just from the High availability for the PKI, using multiple CAs is good way to ensure that your infrastructure can support enterprise scalability.
Such as one offline Root CA,with 2 issue CA in your environment.
Also, one important thing, backup CA, to ensure that the server can be restored from the backup when it is down.
For your reference:
https://social.technet.microsoft.com/wiki/contents/articles/7421.active-directory-certificate-services-ad-cs-public-key-infrastructure-pki-design-guide.aspx#Plan_for_CA_Capacity_Performance_and_Scalability
Best Regards,