I think it would be much easier to cache all sitemaps. For large websites usually, the second level sitemap has 1000 entries while the index might have less than 1000 entries. Thus the second level sitemap could be generated even slower than the index.
On the other topic, I tested this plugin on a multisite installation and it works well, except the CRON jobs must be created for all sites otherwise missing ones will report 404 error on the sitemap index.
I think it would be much easier to cache all sitemaps. For large websites usually, the second level sitemap has 1000 entries while the index might have less than 1000 entries. Thus the second level sitemap could be generated even slower than the index.
On the other topic, I tested this plugin on a multisite installation and it works well, except the CRON jobs must be created for all sites otherwise missing ones will report 404 error on the sitemap index.