Does anyone have any advice for juggling careers?

Here is my advice, don't let your weekend check interfere with your full time check. Sooner or later it will burn you if you let that happen. it is one thing to get called up, it happens. It is another thing to run off and volunteer. Look at it from your civilian employer and your fellow employees. Effectively you are taking away from them the one thing you neither you nor your employer can pay them back...time, time from their family, time from their hobbies, time away from work. You can never pay back that missed anniversary, birthday, kids first homerun, last day with your loved one before they pass.

Get used to having a different sort of life. It's a matter of what you do with the hours in a day. Scheduling is paramount. I went through a period where I was doing full time CS, a lot of extra Reserve hours, and a full time MS program. I have no memory of any time off, sporting event, holiday, etc. between '87-88. I will say that if you can't dump X-Box, put the iPacifier down, turn off the HD, or make excuses for not doing the job and Guard diligently, you'll fail at one of them. Oh, if you have a hobby that takes a chunk of time, it goes into the wait locker too. What hobby you have should revolve around physical fitness.
That said, there's other things to consider. Marriage and kids. Spouses come in two varieties. They either support or don't support your dual career. Ambivalence = Not Support. The later configuration creates a lot of misery. Kids are humans you must commit to and cannot shirk your duty. I don't know how I would have survived '87-88 with kids. I probably wouldn't have that Rug from USC. We adopted later.
COL Dana Hampton makes a good point about USEERA. I've seen spectacular failures because people believe it protects them. It doesn't for most. The reason is DoJ is the decision maker on going after an employer on your behalf. If you take a look at the minimum thresholds of what is worth their time, individuals seldom reach that unless their legislator makes it a PR issue. I got a glimpse first hand with watching an O-5 who worked for me return from the Sandbox only to be denied reemployment. His letter actually stated he was fired because he was a Reservist and couldn't be counted on full time, all the time. DoJ response to this apparent slam dunk? You're an Architect and should have no problem finding employment elsewhere. Also we don't look at anything unless it's worth $2M or more. Business rule. Since that was some time ago, I'd expect the floor to be $4M today. My take is DOJ only supports USEERA when they get something out of it. I don't blame them too much because I never seen Congress appropriate the cash to support the workload. That's why there are mostly insurmountable Business Rules.

Don’t know if this works everywhere but I work In a state prison as well and I know people who are active guard and keep there job in the prison by just showing up once a month or so to keep there peace officer status, and even though they are primarily working for the Guard they can still retire full pension and seniority as me who just works at the facility full time, the state can’t fire you for being in the guard you just have to do so many hours to keep you status Even if you were to deploy you just need orders which would excuse you from not being able to come in for that year but you wouldn’t loose any time toward your pension or seniority, once your on the job best check with some senior COs it the best ways to double dip a retirement just get through your probation period first