Ssl not working with subdomain wordpress multisite

hello folks,

ok so this is my current set up,

a.com >wp main site
b.a.com > wp subdom site

b.a.com exists only insite WP, it is a site from WP, not a wp site by ee.

i have a wildcard ssl setup for a.com and *.a.com

then, i forward
b.com to b.a.com

i have tried both domain aliasing and challenging a second domain in acme.sh settings, but i just cannot get
ssl validation to work correctly

if you goto
b.a.com no SSL errors

if you goto b.com (which fowards to b.a.com)
i get a domain mismatch error,
or when i goto the cert info in url of a browser, i only see *.a.com and a.com as valid domains.

how do i make a cert for b.com also - and have that picked up when you goto the b.a.com wordpress multipress subdomain site.

any ideas?