I only have experience with Atlas flex track but I assume other brands are similar. I remove a tie from each end of the two pieces of flex that are joined. Then I use an
Exacto #17 chisel blade or a single edge razor blade to shave off the tie plates and spikes from the two ties. They then slide easily under the rail joiner. Once it is ballasted, it doesn't move. Usually the downward pressure from the thickness of the joiner keeps it in place anyway, but a spot of Super Glue could fasten any loose ones if needed. As I posted back in post #9, keep the sliding rail to the inside radius of curves, that works best for me. Most people can't see that the tie plates and spikes aren't there, especially if the surrounding area is well scenicked which draws their attention away from the join.