Hi,
In order to check stability for multiple loops, one easy way to do is to use return- ratio(loop gain) analysis, which you have to find a point to break two loops simultaneously and inject a test source. Then you can look for phase margin of the return ratio.
For more details, you can take a look at the following paper
P.J. Hurst and S.H. Lewis, "Determination of Stability Using Return Ratios in Balanced Fully Differential Feedback Circuits," IEEE Trans. on Circuits and Systems II, pp. 805-817, December 1995.