"We are retired and travel so it would be ideal to just have one medical marijuana card that can be used anywhere."
This is a great idea but unfortunately, it is not available at this time. You must have a home state where you collect mail and file taxes so you might contact your local congress person to see their opinion and ask how this could be accomplished. You might also find out what you should do if you are traveling through a state that does not recognize the terrific medical benefits of cannabis and deems it illegal, what are you to do with the cannabis you may have from your 'legal state'? There are so many unanswered questions.