My standard advice to roller noobs (if you are falling off on a regular basis for any reason, you are a noob), is to make sure the front roller is as directly below the front wheel axle as possible without being behind it. This solves 99% of all roller problems and very nearly eliminates the need for the "free motion" rollers in the first place. It is a little counter-intuitive; one thinks you need to "cradle" the bike between the front and rear wheels, but this is unnecessary because there are two rollers already cradling the rear wheel and constraining your fore-aft movement. The front wheel roller plays the roll of the ground and enables you to steer the bike to keep it on the rollers. The ground is usually directly under the front axle, so this is where the roller should go.

FWIW, I ride standard (stationary) rollers almost exclusively indoors in the winter and can ride no handed, take off shirts and stand and even sprint a little standing. When I first started, I had some trouble with staying on the rollers; took a lot of concentration, riding no-handed was basically impossible and even one handed or standing was difficult. I moved the front roller back one "notch" to put it more squarely under my front axle and it was immediately 100% better.