how to write data in two tapes at a time?

If you have only one tape drive it is not physically possible to write to 2 tapes at the same time.

If you want different save sets on two tapes, changing target sessions & parallelism would do it for you.

If you want same data on two different tapes at the same time, you can do it making two pools and groups having the same client. This would be demanding very high resources on client and is not recommended. Instead you can take backup on one tape and clone it immediately after the backup.

In all cases, you need atleast two drives.

I am not sure what your question is.

Here are some possible scenarios in addition to what has already been mentioned.

The 500 GB saveset will write 400 GB to the first tape and then span to a second tape and write the remaining 100 GB. (Assuming no compression)

If the 500 GB saveset is compressible, it may all fit onto one 400 GB tape.

Please clarify the "problem".