I like those 2-in-1 portals - building them involves a lot of small tidbits of maths, to ensure that they work well, but they're a great way to access floating bases without ruining their looks.

Do you mind if I drop a mini-tutorial here on how to do it?

You need to keep track of the following coordinates:

• overworld portal "A" - let's say that it is at (ax, ay, az)
• overworld portal "B" - let's say that it is at (bx, by, bz)
• the block "N" of the Nether portal being used to access "A" - let's say (nx, ny, nz).
• the block "M" of the Nether portal being used to access "B" - let's say (mx, my, mz).

In OP's image, the block "N" would be the block of the Nether portal immediately above the red concrete blocks; it's where your feet would be, once you hop into the portal. While "M" is the one immediately above the blue concrete.

Your Nether portal will link to both A and B if:

• (8nx-ax)² + (ny-ay)² + (8nz-az)² < (8nx-bx)² + (ny-by)² + (8nz-bz)²
• (8mx-bx)² + (my-by)² + (8mz-bz)² < (8mx-ax)² + (my-ay)² + (8mz-az)²

This might look complicated, but it boils down to "N is closer to A than to B; M is closer to B than to A".

Notice the 8's in both equations. They're important there because 1m in the Nether = 8m in the Overworld, but only in the X and Z axes. Don't add those 8's to the Y coordinates!

If you want to be a bit lazy, make sure that both overworld portals share one or two coordinates. For example, if portal A is at (10, 200, -10) and portal B is at (20, 70, -10), then their Z coordinate is the same, and you can simplify the inequations to remove any term with "z" in it. And if the portals were exactly one above another, the inequations collapse to

• (ny-ay)² < (ny-by)²
• (my-by)² < (my-ay)²

Or if they're on the same X and Y coordinates, and only Z is different:

• (8nz-az)² < (8nz-bz)²
• (8mz-bz)² < (8mz-az)²

(If anyone wants, I can help out with the maths for a practical situation.)