As long as your lenses are close to the same throw and light output rating then you can adjust the size of the projection to make the light output equal. A simple way to do this is to calculate lumens per square meter. This is not an accurate unit of light measure but it works for rough calculations.

I am familiar with the x71 at 5500 Lumens.

The x986 I know is only 3300 Lumens but I can only guess the lenses are as are proportionately efficient.

Therefore if you are going to mix projectors, establish the largest screen size for the 986 (for example 1.5m x 2m). Then:

3300 Lu / ( 2m x 1.5m ) = 1100Lu/sqm

So to make your brightness the same, you calculate the reverse of this to get the screen size of the larger projection surfaces.

5500 Lu / 1100Lu/sqm = 5sqm

Since 4:3 use this 5 = (3n) x (4n) = 12nn

Solve for n = sqrt (12/5) = 1.55

Now multiply 1.55 by 3 for height and by 4 for width: 6.20m x 4.65m

So if you had screens roughly these sizes, their brightness would be matched. Use the equations to calculate for yourself.

If however you want to make the screens all the same size, make the top later the black file and adjust it's intensity until the images underneath match in brightness.