You need to basically follow medical O2 storage procedures for small cylinders, Entonox has a few changes, as it needs to stored in slightly warmer conditions as the 2 constituents separate in cold conditions causing problems.
It also needs protecting against theft to prevent misuse.

A quick summary as to how Ambulance services store small quantities at stations is below.

If you need any more info let me know.

ENTONOX cylinders should be:
stored under cover, preferably inside, kept dry and clean
not stored near stocks of combustible materials
not subjected to extremes of heat or cold
stored separately from industrial and other non-medical cylinders stored to maintain separation between full and empty cylinders
used in strict rotation so that cylinders with the earliest filling date are used first
stored separately from other medical cylinders within the store
F size cylinders and larger should be stored vertically. D size cylinders and smaller may be stored horizontally.
Ensure ENTONOX cylinders are maintained at a temperature above 10°C for at least 24 hours before use to ensure the gases are mixed correctly. If this is not possible D, CD and ED size cylinders may be used immediately if inverted three times before use to ensure mixing.
Warning notices prohibiting smoking and naked lights must be posted clearly in the cylinder storage area and the emergency services should be advised of the location of the cylinder store.
Precautions should be taken to protect cylinders from theft. Care is needed when handling and using ENTONOX cylinders.

Phil

Lead
It would only be if you stored the cylinders below the recommended temperature. Good stock rotation should cure that. C, CD, D & E size should be stored horizontally.

That was always the problem with storing liquid air over a long period (large vessels) it used to separate over time.

Worked for major industrial gas supplier in a previous life, working now in medical NHS setting...

O2 is lighter than N2O and will sit at the top when separation occurs. The universe wants chaos and so the gases will tend to mix unless as stated they are stored for prolonged periods at low temps.

The trouble is a patient given unmixed gas will look really happy at first as they breath 100% O2, then half way through the tank a few deep lungfuls of pure N2O and......