Alternate billing identities at Company → NonGST Profiles.
Company → NonGST Profiles (/company/nongst) holds extra letterhead / GST identities under this company (not extra companies).
Who can open it: users granted NonGST Profiles.
Fill the profile name and letterhead fields.
Create Cash / Credit Bills lets you pick the billing company / profile. NonGST profiles hide the GST block on those forms.