Wet layup is preferable. The boiler should be filled completely with water that has your normal chemical treatment. Dry layup will cause more corrosion as it is exposed to the air.
If you want to take one off line for the summer, it should be drained, opened, cleaned and inspected. It can later be filled with treated water while the second is done. I'd rotate infrequently if at all. Leakage will be minimal if the tubes are in good condition.
The American Society of Mechanical Engineers (ASME) publishes the ASME Boiler and Pressure Vessel Code Recommended Guidelines for the Care of Power Boilers. It has everything you need to know.
If you can't find it, contact me and I can copy a couple of pages and mail them to you. Ed