Be sure to bring the fluid in at the bottom of the gland and out the top to prevent an air pocket from forming in the gland. You should be aware that a stationary version is also available from any of the major seal companies. Two way balance is always desirable in any dual seal application to allow you the option of using either a high or low pressure barrier fluid and to prevent the seal faces from opening if either the system or the barrier fluid pressure fluctuates. Be sure to select seal faces with good thermal conductivity. Try to locate any elastomers away from the seal faces if possible.
Elastomers are very sensitive to heat.
This means that an inner seal leak will cause a dilution of your product. There will be no visible evidence of this happening unless someone notices a change in the product concentration or tank level. In operation the outboard seal is carrying the higher differential pressure and should be the first seal to wear out or fail. When this occurs the barrier fluid pressure will drop and the inner seal can blow open. In other words, if the seal works as designed, both seals will fail at the same time.