The wider, 15 second signal you are observing is FT4.
It was meant to be a contesting version of FT8.
It is a faster exchange, wider and efficient, but IMHO totally useless for contesting. Not good in a crowded band...... just like FT8.
Nice mode when too many are on FT8 and I want easy pickings.
They are suppose to operate on 18.104 (FT4).
As of this minute (02:40 UTC) on 20 Meters, each 30 second scroll produces about 5 Chinese, 4 JA's, a couple of Asiatic Russians, a KH6 and a ZL or VK.
Depressing. So goes life.
73 Art K7DWI #2518