Good morning all. I am about to begin laying some engineered bamboo click/lock throughout a two story house. Every bathroom in the house is tile and has an existing marble transition. What would be the best way to make this look like a clean install without ripping up the marble transition?

A T molding may work if you have close to the same height where they meet. You can always cut one side off if you have uneven heights.
A square nose reducer is available, too.This one doesn't taper down.
Both should be available pre-finished to match your flooring.

Are the marble transition and flooring at the same height?

Only in one bathroom (?) The other two bathrooms the tile is 3/8 inch lower than the marble transitions. The marble is 3/4 inch thick.

My apologies...I meant to ask if the marble transitions and new bamboo flooring will be at the same height.

The marble will need a radius on the high edge if it doesn't match the height of the T molding if you need to trim one side, or a bull nose reducer. This will reduce the incident of chipping.