Writing email in Normal Plain Text in Zoho Mail for Desktop, when you change the From Sender, the new signature doesen't replace the old one but is appended to the last one.
This not happens whet you write in RTF: if you change the sender, the new signature replaces the old one.
EXAMPLE (in PLAIN TEXT):
From AA Selected
Appears AA signature
Change From to BB
Appears BB Signature Before AA, with result of viewing both signatures.