GMC is right on.. Over pressuring tanks is mucho quick trip to the morge.

My impression is that fed law requires ALL pressure vessels be labelled with a durable label. That means a rivetted plate or stamped letters in the tank. SO. The rating may be stamped into the tanks somewhere and painted, so look closely.

If you can't figure out the little tank's pressure rating skip it.

You may also have a bit of a circuit problem running them both in the same area.