You and your wife are both submitting applications for a Standard Visitor Visa and you are at the final stage (post biometrics).
do we need to send the application with supporting documents as two
separate postal mails for stamping or can we send it as one?
You can send both applications in a single envelope. Before doing this you should make sure that each application has the other's GWF number on it. The mail room assistant and duty witness will know what to do when your envelope is opened.
Also the return label has to be two different prepaid return shipping
labels or one is enough?
No, one is not enough. Two separate prepaid labels should be created, one for each application. Decisions may be made at different times and evidence may be separated for detailed examination. Consular personnel are smart enough to combine bundles, but Proviso (i.e., their IT system) works best when applications have their own labels.
The unasked question is if you qualify for accelerated service, can it be combined across two applicants? No, just the primary should opt for accelerated service. That's enough to get the entire family a quick decision.