Well, I don´t play professional gigs. I never bring two basses with me.
I never had a problem with my basses on gigs. Playing for more than 10 years, I never broke a string (I have a hard plucking right hand playing), even the old ones.
Once I had problem with an amp (in fact, the sound operator plugged wrong impedance cabs for my head).
What I usually do :
1 - Before leave home to the gig, check all my equipment (Bass, Cables (2 or more), strap, Tuner, Amp, Cabinet). I usually play with the bass I´ll use over the amp I´ll use too.
2 - Always bring an extra battery if I´ll use an active bass (check the battery status first); If possible, replace the battery on bass too (If you didn´t replaced it last month, for example).
3 - On the gig´s place, check local voltage (110 or 220v) before plug the amp on power outlet.
To be very carefull (as I did on last saturday´s gig ), switch the amp to 220v and try to turn it on. If it doesn´t turn on, the local voltage should be 110v, then you switch it to 110v and turn it on. The opposite would be a blew amp/fuse.
4 - Use reliable equipment. It´s better to have one "fault tolerance bass" than two "propably fault basses";
5 - Use a decent gigbag. I have a great one , padded gigbag from Rockbass by Warwick (my basses are Fender Jazz and a Music Man SR5). Usually they still in tune when i get then off the gigbag, but I always check the tune before start sound checking and any set.
6 - Take good care of your gear.
The DI is a good idea. I´ll think about it.