When is it acceptable to post more than one sign on a single post?

Unless the signs are related to each other, all signs should be posted separately and at least 200 feet apart. Note that plaques (designated with a P in the sign designation) are always posted with the sign they supplement and are never posted alone.

Here are some examples from MUTCD Section 2A.16 Standardization of Location.

1. One sign supplements another;
2. Route or directional signs are grouped to clarify information to motorists;
3. Regulatory signs that do not conflict with each other are grouped, such as turn prohibition signs posted with one
   way signs or a parking regulation sign
   posted with a speed limit sign; or
4. Street name signs are posted with a
   stop or yield sign.

For an example please check out the figure 2A-4 from MUTCD Section 2A.16.

References

What is the proper layout and placement for a stop sign and street sign on a single post?
How far apart should signs be placed along the highway?
NYSLTAP- CLRP Quick answer

Traffic Signs and Pavement Markings (pdf)
 

NYS Supplement to the Manual on Uniform Traffic Control Devices (MUTCD)
NYSDOT

May 2015