The basic loops can also be connected using double track. If you have one or more of the switching and crossing track, type K, then you can make the following joins.

The upper join uses L/M-A1-E-K and the lower join comes from one of the loop sides using L/M-E-A-K. Neither are perfect fits. These constructs also enable the use of double curves along in the inner loop.

Joins without track type K are also possible if you use parallel switches F1 and G1. These track types are less common, however.

I am showing K1 track for simplicity, but this is equivalent to a pair of D track if that’s all you have. This type of join is less flexible because it has a more complicated track geometry.