Things to consider. I agree that having two projectors stacked is great for critical images in case one inadvertently loses signal/lamp. Having two projectors will also
gain you the ability to have more options with future productions (two screens,
edge-blending,
etc.). However, maintaining a single
projector will be easier and likely more cost effective.
The actual "brightness" that you will get by stacking two projectors has a number of variables. At full white, you will get close to the combined
intensity of both projectors, but in reality, you won't be just projecting full white. Depending on the technology used to create the
image (
LCD, 1-chip
DLP, 3-chip
DLP,
etc.) the perceived brightness will vary greatly. A 3-panel (
LCD) or 3-chip (
DLP)
projector will have a brighter color spectrum than a 1-chip (
DLP)
projector with the same lumens. This is because the projectors are measured for their brightness at a full white field. This means all light is being projected out. As soon as you color an
image, you no longer are putting out the same amount of light as it is
blocking the light from some of the colors. Depending on the color wheel in a 1-chip
projector, you could have 1/4 the output of the 3-chip
projector of the same lumens.
Now, since you are doing RP, will you be able to project the
image with at least a standard
lens? As soon as you try stacking an
image with short-throw lenses, you will likely run into some extreme challenges.