consider these two as different:
- revisit time: The time a satellite is able to acquire an image over a certain area. This can be of different tracks (flight paths), different looking directions or different viewing angles.
- repeat frequency: this is defined as the time a sensor needs to cover an area at the exact same track. If you want to do interferometry you need a pair of these.
Interferograms result from stereo images while their perpendicular baseline is exactly known. This baseline however should be considerably short (< 500 m). If you have images from differenent tracks the perpendicular baseline is probably several kilometers. You could still stack them by co-registering but the signal is surely off-phase, meaning that interferometry is no longer possible.
Sentinel-1A hat a repeat cycle of 12 days. Now that Sentinel-1B is in orbit, it can be shortened to 6 days. This his however the smallest temporal baseline possible within the same track.