How can you keep Miranda and Jack loyal?

Answer 1

You need to pass either a charm or intimidate check to get both of them loyal, the first time it's easier, if you fail you can come back later for a harder check.

The check is based on how much paragon you have versus how much you could have earned so if you don't mix and match too much during the game and just go for paragon for example you should be able to pass the charm check without any problems, the same goes for the renegade and intimidate options obviously.

if you go 50/50 on paragon and renegade options during your playthrough it will become harder and harder to pass and will at one point or another become impossible.

The exact values for passing individual checks such as these are not known outside bioware but the above explanation is the general principle of it.