If you make a brine and soak your fish, some salt/sugar goes into the fish and it removes some liquid.
Second batch of fish goes into the same brine, it absorbs some salt/sugar and also loses some liquid.
But the second batch of fish will not have the same ratio of salt/sugar the first batch did. It’s physically impossible. First batch already removed some. So the curing process would not be the same.
Also, bacteria may not be present in the first batch, or might start growing halfway through your second batch, or maybe not at all. There’s no way to tell. Until you don’t feel good. Or your buddy. Or wife or kid...
Just saying. Lotta research on this on the net.