One legged pedalling drills. Which, when on the road actually retain both legs guys. You simply focus on using one more than the other. It's only on a trainer that you actually unclip or remove the other leg.

Single legged squats. Which can be performed at home, with no additional equipment.

It also doesn't hurt to examine one's fit and ensure that things are set up for your body. Cleat posititon, saddle height and rotation can all impact on available pedaling force. Most of us have assymetries of various sorts. Depending on the nature and extent of your's some fit adjustments may be warranted.