I think you'd be justified in shooting. Maybe not absolutely legally if they're just there for the money from the register, but I think a Texas jury could be convinced that a reasonable person was in fear of losing their life.
However, being justified (more or less) does not mean it's a good tactical decision.
Priority one is to keep your family and yourself from being killed or seriously injured.
Priority two is to keep anyone else from being killed or seriously injured. (Legally, that includes keeping the bad guys from being injured, if possible. But I doubt that many here have as much concern for the bad guys as they do for innocent bystanders.)
To heck with the fast food joint's money. They can have that. Not my problem. (I don't want to see the bad guys profit from this, nor do I like to see someone wrongfully deprived of money they earned. But it's nowhere near worth risking my life or anyone else's over.)
If I had a clean shot, was extremely confident that I could take them both out without increasing the risk to the people in the resturaunt, I'd probably take it. That's not a suggestion of what anyone else should do.
And the odds of having that opportunity are slim.
So sit back and be a good witness, unless the bad guys make some really stupid tactical mistakes and give me the chance, in which case I might, MIGHT, shoot them.
If they start shooting up the place like the guy at Luby's, then all bets are off. It's open combat and I'm doing my best to put them down.