The answer is easy. Both the Sun and the Moon exert an influence on the waters of the flat Earth, thus there are two high tides each day since both the Sun and the Moon cross the dome once a day.
But the moon and the sun don't cross on the same schedule every day, yet tides are very regular.
If you look at a typical tide chart - you can see that it's a bit more complicated. It is actually the sum of two waves - a large one that follows the position of the moon in the sky - and a much smaller one that follows the sun.
Two important facts:
1) The sun crosses the sky, returning to the same position every 24 hours - but the moon takes about an hour longer (24.83 hours).
2) We always get two high tides per 24.83 hour period - roughly once every twelve
and a half hours.
The FET hypothesis that one high tide is caused by the sun and the other by the moon doesn't work...there are times (eg during a solar eclipse) when the sun and moon are in the same patch of sky. FET would predict that there would be only one double-sized tide that day - and none during the night - but this isn't what happens.
We get TWO high tides every 24.83 hours...regardless of where the sun and moon are - there are ALWAYS two tides, separated separated by the same amount of time.
(Well...approximately. Because the largest effect is from the moon - this fits with the time the moon takes to cross the sky...but the smaller effect of the sun messes with the timing a bit - so the actual highest and lowest point in the cycle varies by maybe +/- 5 minutes depending on the time in the lunar month.)If this FET claim were true then you'd never get a high tide on a moonless night...yet high tides happen at night after the moon has set every single month.
The actual mechanism for tides is a bit more complicated - and it only works in RET.
Because gravity decreases with distance from the source - the gravity of the moon is a little stronger on the side nearest the moon and a little weaker on the opposite side. This tends to stretch the Earth in the direction of the moon. The solid Earth (being solid) doesn't stretch by any noticeable amount - but the oceans are free to do so. This stretching of the oceans in the direction of the moon creates two bulges - one nearest the moon and one on the opposite side. Hence two tides.
FET does not seem to have ANY explanation (certainly none that I've seen posted) for how there can be a high tide in the middle of the night after the moon has set.
Smugglers in the south of England used to time their nefarious landings on nights where the moon had set and the tide was high...this can't happen in FE because the
gravity gravitation of either the sun or the moon is required to create the high tide - and they would both be far away.