Yeah, the main issue with signing up to multiple leagues is knowing what you're getting with each one. Where I lived previously this was particularly tricky on Saturdays because rather than signing up to different areas, I signed up to a "higher" oversubscribed league (changing rooms, respect ropes/marked tech areas, HT drinks and post-game food, paid mileage on top of the standard fee) and a "lower" grassroots league that was always desperate for refs but had none of the above perks.
If I made myself available to the "lower" league I could generally get D1 or Prem matches without too much fuss. Or, I made myself available to the "higher" league and I could get anything from D1 to D5 game, or sometimes nothing. And if I did get nothing and then went back to the "lower" league for a match, they would already have refs assigned to their top divisions and I'd often end up with their D5 games, which could often be fairly grim encounters.
It was also super-easy to accidentally clash matches, so make sure your admin is excellent before trying that!