What if 2 specific subjects must be taught simultaneously?

If these subjects are from different classes check this article:
I need subject Math to be on the same position in classes 5.A and 5.B.

If they are from the same class:
If you have divided class and one part has English and another part have Spanish, the software will automatically put them together if they are from the same division. You do not have to specify this.

OK, but the software puts Math there instead of Spanish!
In this case you have to use another division for math lessons. The software can join any lessons if they are from the same division. See: Divisions

If you do not want to have English alone at the end of the day and Spanish alone at the end of some other day, you might use this: see Both groups have to finish the education at the same time!

Another option is to join the groups or class, but use this only when the join is always the same: How to specify lessons where students are joined from two classes?

For a very special case you can say that groups have to start at the same time: Groups have to start at the same time