My responses below should be read with the knowledge that I am just one person that has gone through the application process for the Air Force. I can't speak to successful JAG-attorney married couples, unfortunately.
I was fortunate enough to get a JAG internship this summer (I am a rising 2L); does this improve my chances?
The consensus on this seems to be yes. It demonstrates interest and some commitment. How much does it help? I can't say. The help from just the line item on your resume is nice, but you might find that the best benefits will be in what you can get out of the internship. You will hopefully secure several very solid letters of recommendation from people who can speak to what it takes to make a good officer better than about any non-military reference you can get. You will also meet a lot of people that have gone through the process and can give you advice on what might help in getting selected. You will also have a chance to see for yourself what matters on the job (to an extent), and that can help you mold your choices throughout your remaining time in law school and in putting your application package together.
Should I try to intern with JAG my 2L summer as well, or would that be useless?
Two summers with a JAG corps might not be twice as good as one, but it can certainly help. Consider everything I mentioned above and that you will hopefully have an entire new base of people to learn from and possibly get letters of recommendation from. It shows further interest and commitment to government/military work too.
In the event that I don't get selected for boards I obviously don't want to be jobless, so I don't know if my best bet is to look for a firm job for next summer.
This is one of the most difficult parts of the process to me - figuring out a plan B if it doesn't work out. I don't have any really good advice. It was easier for me for two reasons. First, I worked full-time through law school the first 3 years, and I kept my job part-time my last year of law school, so I had my foot in the water of my old career in case things didn't work out. Second, I found that with respect to non-JAG attorney jobs, I only wanted PD/DA kind of work. It's easier to mold yourself for that as your back-up plan than firm life. All I can offer is to wish you good luck in deciding.