You can send the ringtone you want as mp3 or ogg file attached to a WhatsApp message from your primary WhatsApp account to your secondary Whatsapp account. In the secondary WhatsApp save it to the folder
Ringtones on the internal sd-card section (should be path
Afterwards your system should be able to detect and use the saved ringtone. May be a reboot is required if the ringtone does not directly appear in the ringtone list.